1. Introduction to Metamorphism
IN THIS LECTURE
Definition of Metamorphism
Limits of Metamorphism
Agents of Metamorphic Change
Temperature
Pressure
Stress and Strain
Fluids
Metamorphic Change
Classification of Metamorphic Rocks
Types of Metamorphism

2. Introduction to Metamorphism
Rocks as chemical systems represented by a particular assemblage of coexisting phases (thermodynamic equilibrium and the governed by the phase rule)
A basaltic composition can be either:
Melt
Cpx + plag ( olivine, ilmenite…)
Or any combination of melt + minerals along the liquid line of descent
If uplifted and eroded  surface, will weather  a combinations of clays, oxides…
Between these is the realm of metamorphism
We shall see that the chemistry of a basalt can  a number of different mineral assemblages between the conditions under which plag + cpx and clays + oxides are stable
The mineralogy is dependent upon P, T, and X

3. Introduction to 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

4. The Limits of Metamorphism
Low-temperature limit grades into diagenesis
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
Metamorphism begins in the range of oC for the more unstable types of protolith
Marked by the formation of minerals such as laumontite, analcime, heulandite, carpholite, paragonite, prehnite, pumpellyite, lawsonite, glaucophane or stilpnomelane

5. The Limits of Metamorphism
High-temperature limit grades into melting
Over the melting range solids and liquids coexist
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
Migmatites (“mixed rocks”) are gradational
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

6. The Limits of Metamorphism
Migmatites: metamorphic or igneous rocks?

7. Limits of Metamorphism

8. Agents of Metamorphic Change
There are several main agents of metamorphic change
Temperature
Pressure
Stress and Strain
Fluids

9. Agents of Metamorphic Change 1. Temperature
TEMPERATURE: typically the most important factor in metamorphism
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,
Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements

10. Agents of Metamorphic Change 1. Temperature
Increasing temperature has several effects
Promotes recrystallization leading to increased grain size
Larger surface/volume ratio of a mineral means lower stability
Increasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grains
Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditions
Overcomes kinetic barriers that might otherwise preclude the attainment of equilibrium
Especially for fine-grained and unstable materials in a static environment (shear stresses often reduce grain size)

11. Agents of Metamorphic Change 2. Pressure
Normal gradients may be perturbed in several ways
The two main examples are
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”

12. 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

13. Agents of Metamorphic Change 3. Stress and Strain
Stress is an applied force acting on a rock (over a particular cross-sectional area)
Strain is the response of the rock to an applied stress (= yielding or deformation)
Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage
Strain energy may overcome kinetic barriers to reactions

14. Agents of Metamorphic Change 4. Fluids
Evidence for the existence of a metamorphic fluid
Fluid inclusions
Fluids are required for hydrous or carbonate phases
Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures
Pfluid indicates the total fluid pressure, which is the sum of the partial pressures of each component (Pfluid = pH2O + pCO2 + …)
May also consider the mole fractions of the components, which must sum to 1.0 (XH2O + XCO2 + … = 1.0)

15. Metamorphic Change Metamorphic grade
A general increase in degree of metamorphism without specifying the exact relationship between temperature and pressure
Generally both temperature and pressure increase
Three terms used
Low Grade
Medium Grade
High Grade
We may thus refer to “high-grade” rocks or “low-grade” rocks from any area depicted in Fig
Consider pressure as a modifier, in the sense that temperature can increase along any number of pressure-varied paths
High T/P paths (low P) favor the formation of low-density metamorphic minerals as temperature rises
Low T/P paths (high P) favor denser minerals

16. The Progressive Nature of Metamorphism
PROGRADE: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions
Prograde metamorphism: changes in a rock that accompany increasing metamorphic grade
RETROGRADE: decreasing grade as rock cools and recovers from a metamorphic or igneous event
Retrograde metamorphism: any changes that accompany decreasing metamorphic grade

17. Evidence for Metamorphic Change
How do we see metamorphic change
Mineral assemblage and grainsize can be used to estimate metamorphic grade
Gradients in T, P, Xfluid across an area
Zonation in the mineral assemblages
Gradients in temperature, pressure, and fluid composition across an area are the norm
As a result, zonation in the mineral assemblages constituting the rocks that equilibrate spanning these gradients
Along a traverse in an eroded metamorphic area cross from non- metamorphosed rocks through zones of progressively higher metamorphic grade (or through zones reflecting metasomatic composition gradients)

18. Classification of Metamorphic Rocks
Two main ways in which metamorphic rocks are classified
Based on the style or type of metamorphism
Based on the mineral assemblage or texture of the rocks
We’ll look first at the types of metamorphism

19. The Types of Metamorphism
Different approaches to classification based on type
Based on principal process or agent
Dynamic Metamorphism
Thermal Metamorphism
Dynamo-thermal Metamorphism
Dynamic Metamorphism when deviatoric stress is dominant and deformation + recrystallization is the main process
Thermal Metamorphism when temperature in a near-static stress field is the main agent
Dynamo-thermal Metamorphism when both temperature and deformation are prevalent

20. The Types of Metamorphism
2. Based on setting
Contact Metamorphism
Pyrometamorphism
Regional Metamorphism
Orogenic Metamorphism
Burial Metamorphism
Ocean Floor Metamorphism
Hydrothermal Metamorphism
Fault-Zone Metamorphism
Impact or Shock Metamorphism
We’ll look at the classifivation based on setting

21. Contact Metamorphism Adjacent to igneous intrusions
Result of thermal (and possibly metasomatic – involvement of fluids) effects of hot magma intruding cooler shallow rocks
Occur over a wide range of pressures, including very low pressures
Usually leads to the formation of a contact aureole around the pluton. A contact aureole is a zone in which the rocks into which the pluton has intruded have been affected by the heat of the intrusion and have developed new metamorphic mineral assemblages
Plutons can rise and transmit heat to the shallow crust, so may occur over a wide range of pressures, including very low

22. Contact Metamorphism The size and shape of an aureole is controlled by
The nature of the pluton, ie
Size
Shape
Orientation
Composition
Temperature
The nature of the country rocks
Depth and metamorphic grade prior to intrusion
Permeability

23. Contact Metamorphism
Most easily recognized where a pluton is introduced into shallow rocks in a static environment
The rocks near the pluton are often high-grade rocks with an isotropic fabric: hornfelses (or granofelses) in which relict textures and structures are common
Polymetamorphic rocks are common, usually representing an orogenic event followed by a contact one, e.g. spotted phyllite or slate
Overprint may be due to:
Lag time between the creation of the magma at depth during T maximum, and its migration to the lower grade rocks above
Plutonism may reflect a separate phase of post-orogenic collapse magmatism

24. Contact Metamorphism Pyrometamorphism
Very high temperatures at very low pressures, generated by a volcanic or subvolcanic body
Also developed in xenoliths
Not very common and won’t be looked at further in this course

25. Regional Metamorphism
Regional Metamorphism sensu lato
metamorphism that affects a large body of rock, and thus covers a great lateral extent
Three principal types
Orogenic metamorphism
Burial metamorphism
Ocean-floor metamorphism

26. Regional Metamorphism
OROGENIC METAMORPHISM
Type of metamorphism associated with convergent plate margins
Dynamo-thermal, involving one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress)
Foliated rocks are a characteristic product

27. Regional Metamorphism OROGENIC METAMORPHISM
Figure Schematic model for the sequential (a  c) development of a “Cordilleran-type” or active continental margin orogen. The dashed and black layers on the right represent the basaltic and gabbroic layers of the oceanic crust. From Dewey and Bird (1970) J. Geophys. Res., 75, ; and Miyashiro et al. (1979) Orogeny. John Wiley & Sons.
(a) = the incipient stages of subduction
(b) “orogenic welt” created by compression, crustal thickening, thrust stacking of oceanic slices, and addition of magmatic material from below
Underthrusting in the forearc migrates trenchward, adding successive slabs to the base of the outer welt (tectonic underplating)
Heat added by rising plutons, magmatically underplated magma, and induced mantle convection
Temperature increases both downward and toward the axial portion of the welt where plutons concentrated

28. Regional Metamorphism
OROGENIC METAMORPHISM
Uplift and erosion
Metamorphism often continues after major deformation ceases
Metamorphic pattern is simpler than the structural one
Pattern of increasing metamorphic grade from both directions toward the core area
Most orogenic belts have several episodes of deformation and metamorphism, creating a more complex polymetamorphic pattern
Associated with continental collision

29. Regional Metamorphism
OROGENIC METAMORPHISM
Batholiths are usually present in the highest grade areas
If plentiful and closely spaced, may be called regional contact metamorphism

30. Regional Metamorphism
BURIAL METAMORPHISM
Low grade metamorphism in sedimentary basins due to burial
Example: Southland Syncline in New Zealand
A thick pile (> 10 km) of Mesozoic volcaniclastics had accumulated
Mild deformation and no igneous intrusions discovered
Fine-grained, high-temperature phases, glassy ash: very susceptible to metamorphic alteration
Metamorphic effects attributed to increased pressure and temperature due to burial
Range from diagenesis to the formation of zeolites, prehnite, pumpellyite, laumontite, etc.
A term coined by Coombs (1961) for low-grade metamorphism that occurs in sedimentary basins due to burial by successive layers
Coombs worked in the Southland Syncline in New Zealand, where a thick pile (> 10 km) of Mesozoic volcaniclastics had accumulated

31. Regional Metamorphism
BURIAL METAMORPHISM
Occurs in areas that have not experienced significant deformation or orogeny
Restricted to large, relatively undisturbed sedimentary piles away from active plate margins
The Gulf of Mexico
Bengal Fan
It is thus restricted to large, relatively undisturbed sedimentary piles away from active plate margins
The Gulf of Mexico may represent a modern example
Bengal Fan is another, fed by the Ganges and Brahmaputra rivers, has the form of a sedimentary wedge accumulating along a passive continental margin

32. Regional Metamorphism
BURIAL METAMORPHISM
Bengal Fan represents a sedimentary pile > 22 km
Extrapolating implies oC at the base (P ~ 0.6 GPa)
Well into the metamorphic range, and the weight of the overlying sediments is sufficient to impart a foliation at depth
Passive margins often become active
Areas of burial metamorphism may thus become areas of orogenic metamorphism
Seismic data in Bengal Fan  sedimentary pile > 22 km
Extrapolating the low geothermal gradient at the surface (18-22oC/km)  oC at the base (P ~ 0.6 GPa)
Conditions are well into the metamorphic range, and the weight of the overlying sediments  sufficient compression to impart a foliation to the metamorphic rocks forming at depth
It may be impossible to distinguish a hand specimen of a rock retrieved from these depths with one from the lower grade regions of an orogenic belt
End: Typical examples of burial and regional metamorphism are easily recognized today, but transitional types are common

33. Regional Metamorphism
OCEAN-FLOOR METAMORPHISM
Affects the oceanic crust at ocean ridge spreading centres
Wide range of temperatures at relatively low pressure, beginning in the diagenesis field and increasing to lower greenschist facies
Metamorphic rocks exhibit considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na
These changes can be correlated with exchange between basalt and hot seawater
We’ve seen this already when we looked at the Cyprus thin-sections!
Variety of metamorphic minerals in ocean-floor rocks, representing a wide range of temperatures at relatively low pressure
Alteration concentrated along vein systems, presumably associated with hydrothermal activity (note black smokers)
Seawater penetrates down ubiquitous fracture systems, where it becomes heated, and leaches metals and silica from the hot basalts

34. Regional Metamorphism
OCEAN-FLOOR METAMORPHISM
May be considered another example of hydrothermal metamorphism
Highly altered chlorite-quartz rocks- distinctive high-Mg, low- Ca composition
The hot water circulates convectively back upward, exchanging components with the rocks – hydrothermal met
Alteration occurs near the ridge where magmatism and heat is concentrated
Regional in the sense that it affects virtually all of the oceanic crust, but really a more localized process
Highly altered chlorite-quartz rocks have a distinctive high-Mg, low-Ca composition that is unlike any other known sedimentary or igneous rock
These rocks are a prime candidate for the protolith of the unusual cordierite-anthophyllite rocks found at higher grades of metamorphism in some orogenic areas