2. The IUGS-SCMR proposed this definition:
“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.”
Chapter 21: Metamorphism
3. The Limits of Metamorphism
Low-temperature limit grades into diagenesis
• Processes are indistinguishable
• Metamorphism begins in the range of 100-150oC
for the more unstable types of protolith
• Some zeolites are considered diagenetic and
others metamorphic – pretty arbitrary
4. 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
5. Metamorphic Agents and Changes
• Temperature: typically
the most important factor
in metamorphism
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,
269-311.
6. Metamorphic Agents and Changes
Increasing temperature has several effects
1) Promotes recrystallization increased grain
size
2) Drive reactions (endothermic)
3) Overcomes kinetic barriers
7. Metamorphic Agents and Changes
Pressure
• “Normal” gradients perturbed in several ways,
most commonly:
High T/P geotherms in areas of plutonic
activity or rifting
Low T/P geotherms in subduction zones
8. Figure 21.1. 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.
9. Metamorphic Agents and Changes
• Metamorphic grade: a general increase in
degree of metamorphism without specifying
the exact relationship between temperature
and pressure
10. Metamorphic Agents and Changes
• Lithostatic pressure - uniform stress (hydrostatic)
• Deviatoric stress = pressure unequal in different
directions
• Resolved into three mutually perpendicular stress
(s) components:
s1 is the maximum principal stress
s2 is an intermediate principal stress
s3 is the minimum principal stress
• In hydrostatic situations all three are equal
11. Metamorphic Agents and Changes
• Stress
• Strain deformation
• Deviatoric stress affects the textures and
structures, but not the equilibrium mineral
assemblage
• Strain energy may overcome kinetic barriers to
reactions
12. • Foliation is a common result, which allows us to
estimate the orientation of s1
s1 > s2 = s3 foliation and no lineation
s1 = s2 > s3 lineation and no foliation
s1 > s2 > s3 both foliation and lineation
Figure 21.3. 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.
s1
Strain
ellipsoid
13. Metamorphic Agents and Changes
Shear motion occurs along planes at an angle to s1
Figure 21.2. 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.
s1
14. Metamorphic Agents and Changes
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
15. Metamorphic Agents and Changes
• Pfluid = S partial pressures of each
component (Pfluid = pH2O + pCO2
+ …)
• Mole fractions of components must sum to
1.0 (XH2O + XCO2
+ … = 1.0)
• pH2O = XH2O x Pfluid
• Gradients in T, P, Xfluid
• Zonation in mineral assemblages
16. The Types of Metamorphism
Different approaches to classification
1. Based on principal process or agent
• Dynamic Metamorphism
• Thermal Metamorphism
• Dynamo-thermal Metamorphism
17. The Types of Metamorphism
Different approaches to classification
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
18. The Types of Metamorphism
Contact Metamorphism
The size and shape of an aureole is controlled by:
• The nature of the pluton
• The nature of the country rocks
Size
Shape
Orientation
Temperature
Composition
Composition
Depth and metamorphic grade prior to intrusion
Permeability
19. Contact Metamorphism
• Adjacent to igneous intrusions
• Thermal (± metasomatic) effects of hot magma
intruding cooler shallow rocks
• Occurs over a wide range of pressures, including
very low
• Contact aureole
20. The Types of Metamorphism
Contact Metamorphism
Most easily recognized where a pluton is introduced into
shallow rocks in a static environment
Hornfelses (granofelses) commonly with relict
textures and structures
21. The Types of Metamorphism
Contact Metamorphism
Polymetamorphic rocks are common, usually
representing an orogenic event followed by a
contact one
• Spotted phyllite (or slate)
• Overprint may be due to:
Lag time for magma migration
A separate phase of post-orogenic collapse
magmatism (Chapter 18)
22. The Types of Metamorphism
Pyrometamorphism
Very high temperatures at low pressures,
generated by a volcanic or sub-volcanic body
Also developed in xenoliths
23. The Types of 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
24. The Types of Metamorphism
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
25. The Types of Metamorphism
Orogenic
Metamorphism
Figure 21.6. 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, 2625-2647;
and Miyashiro et al. (1979)
Orogeny. John Wiley & Sons.
27. The Types of 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
From Understanding
Earth, Press and Siever.
Freeman.
28. The Types of Metamorphism
Orogenic Metamorphism
• Polymetamorphic patterns
• Continental collision
• Batholiths are usually present in the highest grade areas
• If plentiful and closely spaced, may be called regional
contact metamorphism
29. The Types of Metamorphism
Burial metamorphism
• Southland Syncline in New Zealand: thick pile (> 10 km)
of Mesozoic volcaniclastics
• Mild deformation, no igneous intrusions discovered
• Fine-grained, high-temperature phases, glassy ash: very
susceptible to metamorphic alteration
• Metamorphic effects attributed to increased temperature
and pressure due to burial
• Diagenesis grades into the formation of zeolites, prehnite,
pumpellyite, laumontite, etc.
30. The Types of Metamorphism
Hydrothermal metamorphism
• Hot H2O-rich fluids
• Usually involves metasomatism
• Difficult type to constrain: hydrothermal effects
often play some role in most of the other types of
metamorphism
31. The Types of 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?
32. The Types of Metamorphism
Burial metamorphism occurs in areas that have not
experienced significant deformation or orogeny
• Bengal Fan sedimentary pile > 22 km
• Extrap. 250-300oC at the base (P ~ 0.6 GPa)
• Passive margins often become active
• Areas of burial metamorphism may thus become
areas of orogenic metamorphism
33. The Types of Metamorphism
Ocean-Floor Metamorphism affects the oceanic
crust at ocean ridge spreading centers
• Considerable metasomatic alteration, notably loss
of Ca and Si and gain of Mg and Na
• Highly altered chlorite-quartz rocks- distinctive
high-Mg, low-Ca composition
• Exchange between basalt and hot seawater
• Another example of hydrothermal metamorphism
34. The Types of Metamorphism
Impact metamorphism at meteorite (or other
bolide) impact craters
Both correlate with dynamic metamorphism,
based on process
Fault-Zone and Impact Metamorphism
High rates of deformation and strain with only
minor recrystallization
35. (a) Shallow fault
zone with fault
breccia
(b) Slightly deeper
fault zone (exposed
by erosion) with
some ductile flow
and fault mylonite
Figure 21.7. Schematic cross
section across fault zones. After
Mason (1978) Petrology of the
Metamorphic Rocks. George Allen
& Unwin. London.
36. Prograde 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 accompanying
changes
37. The Progressive Nature of Metamorphism
A rock at a high metamorphic grade probably
progressed through a sequence of mineral
assemblages rather than hopping directly from an
unmetamorphosed rock to the metamorphic rock
that we find today
38. The Progressive Nature of Metamorphism
Retrograde metamorphism typically of minor
significance
• Prograde reactions are endothermic and easily
driven by increasing T
• Devolatilization reactions are easier than
reintroducing the volatiles
• Geothermometry indicates that the mineral
compositions commonly preserve the maximum
temperature
39. 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
40. Why Study Metamorphism?
• Interpretation of the conditions and evolution of
metamorphic bodies, mountain belts, and ultimately the
state and evolution of the Earth's crust
• Metamorphic rocks may retain enough inherited
information from their protolith to allow us to interpret
much of the pre-metamorphic history as well
41. Orogenic Regional Metamorphism of
the Scottish Highlands
• George Barrow (1893, 1912)
• SE Highlands of Scotland - Caledonian Orogeny
~ 500 Ma
• Nappes
• Granites
42. Barrow’s
Area
Figure 21.8. 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.
43. 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
44. 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, uartz, plagioclase, garnet, and perhaps staurolite. Some
kyanite may also be present (although kyanite and sillimanite are
both polymorphs of Al2SiO5)
45. • Sequence = “Barrovian zones”
• The P-T conditions referred to as “Barrovian-type”
metamorphism (fairly typical of many belts)
• Now extended to a much larger area of the Highlands
• Isograd = line that separates the zones (a line in the field
of constant metamorphic grade)
46. Figure 21.8. 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.
47. To summarize:
• An isograd represents the first appearance of a particular
metamorphic index mineral in the field as one progresses
up metamorphic grade
• When one crosses an isograd, such as the biotite isograd,
one enters the biotite zone
• Zones thus have the same name as the isograd that forms
the low-grade boundary of that zone
• Because classic isograds are based on the first appearance
of a mineral, and not its disappearance, an index mineral
may still be stable in higher grade zones
48. A variation occurs in the area just to the north of
Barrow’s, in the Banff and Buchan district
• Pelitic compositions are similar, but the sequence
of isograds is:
chlorite
biotite
cordierite
andalusite
sillimanite
49. The stability field of andalusite occurs at pressures less than
0.37 GPa (~ 10 km), while kyanite sillimanite at the
sillimanite isograd only above this pressure
Figure 21.9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and
sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the
presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).
50. Regional Burial Metamorphism
Otago, New Zealand
• Jurassic graywackes, tuffs, and volcanics in a deep
trough metamorphosed in the Cretaceous
• Fine grain size and immature material is highly
susceptible to alteration (even at low grades)
51. Regional Burial Metamorphism
Otago, New Zealand
Section X-Y shows more detail
Figure 21.10. Geologic sketch map of the South Island of New
Zealand showing the Mesozoic metamorphic rocks east of the
older Tasman Belt and the Alpine Fault. The Torlese Group is
metamorphosed predominantly in the prehnite-pumpellyite
zone, and the Otago Schist in higher grade zones. X-Y is the
Haast River Section of Figure 21-11. From Turner (1981)
Metamorphic Petrology: Mineralogical, Field, and Tectonic
Aspects. McGraw-Hill.
52. Regional Burial Metamorphism
Otago, New Zealand
Isograds mapped at the lower grades:
1) Zeolite
2) Prehnite-Pumpellyite
3) Pumpellyite (-actinolite)
4) Chlorite (-clinozoisite)
5) Biotite
6) Almandine (garnet)
7) Oligoclase (albite at lower grades is replaced by a
more calcic plagioclase)
53. Regional Burial Metamorphism
Figure 21.11. Metamorphic zones of the Haast
Group (along section X-Y in Figure 21-10). After
Cooper and Lovering (1970) Contrib. Mineral.
Petrol., 27, 11-24.
54. Paired Metamorphic Belts of Japan
Figure 21.12. The Sanbagawa and Ryoke
metamorphic belts of Japan. From Turner
(1981) Metamorphic Petrology:
Mineralogical, Field, and Tectonic Aspects.
McGraw-Hill and Miyashiro (1994)
Metamorphic Petrology. Oxford University
Press.
56. Figure 21.13. Some of the
paired metamorphic belts
in the circum-Pacific
region. From Miyashiro
(1994) Metamorphic
Petrology. Oxford
University Press.
57. Contact Metamorphism of Pelitic Rocks
in the Skiddaw Aureole, UK
• Ordovician Skiddaw Slates (English Lake District)
intruded by several granitic bodies
• Intrusions are shallow
• Contact effects overprinted on an earlier low-grade
regional orogenic metamorphism
58. Contact Metamorphism of Pelitic Rocks
in the Skiddaw Aureole, UK
• The aureole around the Skiddaw granite was sub-
divided into three zones, principally on the basis of
textures:
• Unaltered slates
• Outer zone of spotted slates
• Middle zone of andalusite slates
• Inner zone of hornfels
• Skiddaw granite
Increasing
Metamorphic
Grade
Contact
59. Figure 21.14. Geologic
Map and cross-section of
the area around the
Skiddaw granite, Lake
District, UK. After
Eastwood et al (1968).
Geology of the Country
around Cockermouth and
Caldbeck. Explanation
accompanying the 1-inch
Geological Sheet 23, New
Series. Institute of
Geological Sciences.
London.
60. Contact Metamorphism of Pelitic Rocks
in the Skiddaw Aureole, UK
• Middle zone: slates more thoroughly recrystallized, contain
biotite + muscovite + cordierite + andalusite + quartz
Figure 21.15. Cordierite-
andalusite slate from the
middle zone of the Skiddaw
aureole. From Mason (1978)
Petrology of the
Metamorphic Rocks. George
Allen & Unwin. London.
1 mm
61.
62. Contact Metamorphism of Pelitic Rocks
in the Skiddaw Aureole, UK
Inner zone:
Thoroughly recrystallized
Lose foliation
Figure 21.16. Andalusite-cordierite
schist from the inner zone of the
Skiddaw aureole. Note the chiastolite
cross in andalusite (see also Figure 22-
49). From Mason (1978) Petrology of
the Metamorphic Rocks. George Allen &
Unwin. London.
1 mm
63. Contact Metamorphism of Pelitic Rocks
in the Skiddaw Aureole, UK
• The zones determined on a textural basis
• Prefer to use the sequential appearance of
minerals and isograds to define zones
• But low-P isograds converge in P-T
• Skiddaw sequence of mineral development with
grade is difficult to determine accurately
64. Contact Metamorphism and Skarn
Formation at Crestmore, CA, USA
• Crestmore quarry in the Los Angeles basin
• Quartz monzonite porphry intrudes Mg-bearing
carbonates (either late Paleozoic or Triassic)
• Burnham (1959) mapped the following zones and the
mineral assemblages in each (listed in order of increasing
grade):
66. Contact Metamorphism and Skarn
Formation at Crestmore, CA, USA
An idealized cross-section through the aureole
Figure 21.17.
Idealized N-S cross
section (not to scale)
through the quartz
monzonite and the
aureole at Crestmore,
CA. From Burnham
(1959) Geol. Soc.
Amer. Bull., 70, 879-
920.
67. Contact Metamorphism and Skarn
Formation at Crestmore, CA, USA
1. The mineral associations in successive zones (in all
metamorphic terranes) vary by the formation of new
minerals as grade increases
This can only occur by a chemical reaction in which some
minerals are consumed and others produced
68. Contact Metamorphism and Skarn
Formation at Crestmore, CA, USA
a) Calcite + brucite + clinohumite + spinel zone to the
Calcite + clinohumite + forsterite + spinel sub-zone
involves the reaction:
2 Clinohumite + SiO2 9 Forsterite + 2 H2O
b) Formation of the vesuvianite zone involves the reaction:
Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite
+ 15 SiO2 + 12 H2O 6 Vesuvianite + 2 CO2
69. Contact Metamorphism and Skarn
Formation at Crestmore, CA, USA
2) Find a way to display data in simple, yet useful ways
If we think of the aureole as a chemical system, we note
that most of the minerals consist of the components
CaO-MgO-SiO2-CO2-H2O (with minor Al2O3)
70. Zones are numbered
(from outside inward)
Figure 21.18. CaO-MgO-SiO2 diagram at a fixed
pressure and temperature showing the
compositional relationships among the minerals
and zones at Crestmore. Numbers correspond to
zones listed in the text. After Burnham (1959) Geol.
Soc. Amer. Bull., 70, 879-920; and Best (1982)
Igneous and Metamorphic Petrology. W. H.
Freeman.
71. Figures not used
Figure 21.4. A situation in which lithostatic
pressure (Plith) exerted by the mineral grains
is greater than the intergranular fluid
pressure (Pfluid). At a depth around 10 km
(or T around 300oC) minerals begin to yield
or dissolve at the contact points and shift
toward or precipitate in the fluid-filled
areas, allowing the rock to compress. The
decreased volume of the pore spaces will
raise Pfluid until it equals Plith. Winter (2001)
An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
72. Figures not used
Figure 21.5. Temperature distribution within a 1-km thick vertical dike and in the country rocks (initially at 0oC) as a function of time. Curves are
labeled in years. The model assumes an initial intrusion temperature of 1200oC and cooling by conduction only. After Jaeger, (1968) Cooling and
solidification of igneous rocks. In H. H. Hess and A. Poldervaart (eds.), Basalts, vol. 2. John Wiley & Sons. New York, pp. 503-536.
Editor's Notes
Rocks as chemical systems (Ch. 5)
-> a particular assemblage of coexisting phases (thermodynamic equilibrium and 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
Deformation alone does not count: requires crystallization or recrystallization
The boundary is somewhat arbitrary
Diagenetic/weathering processes are indistinguishable from metamorphic
Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
Metamorphism begins in the range of 100-150oC 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
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
Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements
1) Promotes recrystallization increased grain size
Larger surface/volume ratio of a mineral lower stability
Fine aggregates coalesce to larger grains
Especially for fine-grained and unstable materials in a static environment (shear stresses often reduce grain size)
2) Drives reactions that consume 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
Temperature rarely increases without an accompanying increase in pressure (geothermal gradients)
Most disturbances are transient and eventually return to “normal”
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
We may thus refer to “high-grade” rocks or “low-grade” rocks from any area depicted in Fig. 21-1
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
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 can be maintained only if application keeps pace with the tendency of the rock to yield
This occur most often in orogenic belts, extending rifts, or in shear zones. (i.e. generally at or near plate boundaries)
Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage
Strain energy may overcome kinetic barriers to reactions
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
Metamorphic fluids dominated by H2O, but CO2 may also be present in rocks with a significant carbonate
CH4, S, and N2 may be minor components, as well as dissolved species, notably alkalis and halides
Nature and source of fluids? Are they present at the higher metamorphic grades?
Fluids can be meteoric, juvenile magmatic, subducted material, trapped sedimentary brines, or degassing of the mantle
The motion of fluids may transport various chemical species over considerable distances- metasomatism
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)
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
Plutons can rise and transmit heat to the shallow crust, so may occur over a wide range of pressures, including very low
The foliated phyllite formed during a regional event and the later ovoid “spots” are minerals that grew during the contact event
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 (Chapter 18)
A minor type of contact metamorphism characterized by very high temperatures at very low pressures, generated by a volcanic or subvolcanic body
Also developed in xenoliths
Pyrometamorphism may be accompanied by various degrees of partial melting
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”
(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
Temperature increases both downward and toward the axial portion of the welt where plutons concentrated
Uplift and erosion results in exposure of the metamorphic rocks
Heat dissipates slowly, so the metamorphism often continues after major deformation ceases
When this occurs, the metamorphic pattern is simpler than the structural one
Folding and thrusting are often complex, but the metamorphic pattern may be a simple thermal dome, centering on the metamorphic/igneous core where heat input, thickening, and uplift are the greatest
Exposed surface 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
Continental collision involves interaction of a continental arc with a continental mass having a “passive” margin and an apron of sediments extending from the continental shelf. Such collisions will usually produce even more complex structural, magmatic, and metamorphic patterns
Although batholiths are usually present in the highest grade areas of regional terranes, the metamorphism isn’t considered contact metamorphism because it develops regionally, and the pattern of metamorphic grade does not relate directly to the proximity of the igneous contacts
Contact metamorphism typically occurs locally within regional terranes
In many cases intrusive rocks may be plentiful and closely spaced, so that it is difficult or impossible to distinguish regional metamorphism from overlapping contact aureoles. Spear (1993) calls such situations regional contact metamorphism
Burial metamorphism = for low-grade metamorphism in sedimentary basins due to burial
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
Coombs (1961) also proposed hydrothermal metamorphism
Many areas of burial metamorphism are also hydrothermal fields, suggesting an elevated geothermal gradient and fluids may be important elements in at least some cases of burial metamorphism
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
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
Seismic data in Bengal Fan sedimentary pile > 22 km
Extrapolating the low geothermal gradient at the surface (18-22oC/km) 250-300oC 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
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
“Fault” includes zones of distributed shear that can be up to several kilometers across
Impact metamorphism (also called shock metamorphism) occurs at meteorite (or other bolide) impact craters
Both fault-zone and impact metamorphism correlate with dynamic metamorphism, based on process
Schematic cross-section across fault zones
Metamorphic rocks usually maintain equilibrium as grade increases
High-grade metamorphic rock probably progressed through a sequence of mineral assemblages as it adjusted to increasing temperature and pressure, rather than hopping directly from un-met to the metamorphic rock that we find today
If a metamorphosed sedimentary rock experienced a cycle of increasing metamorphic grade, followed by decreasing grade, at what point on this cyclic P-T-t path did its present mineral assemblage last equilibrate?
The zonal distribution of metamorphic rock types preserved in a geographic sequence of increased metamorphic grade suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature) experienced by that rock during metamorphism
Retrograde is usually detectable by observing textures, such as the incipient replacement of high-grade minerals by low-grade ones at their rims
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
George Barrow (1893, 1912): one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphism
Caledonian orogeny, ~500 Ma ago
Deformation was intense: rocks folded into a series of nappes
Numerous granites also intruded toward the end of the orogeny, after main regional metamorphism
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
This sequence of zones now recognized in other orogenic belts, and is now so well established in the literature that the zones are often referred to as the Barrovian zones
Tilley, Kennedy, etc. confirmed Barrow’s zones, and extended them over a much larger area of the Highlands
Tilley coined the term isograd for the line that separates the zones
An isograd, then, is meant to indicate a line in the field of constant metamorphic grade
Really = the intersection of the isogradic surface with the Earth’s surface
Later we shall see broader categories: metamorphic facies
Barrovian zones have become the norm to which we compare all other areas of regional metamorphism
OK practice, but we shouldn’t let these zones constrain our thinking or our observations
Other zones may be important and useful locally
A chloritoid zone is prevalent in the Appalachians (X)
The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineral
The geothermal gradient in this northern district was higher than in Barrow’s area, and rocks at any equivalent temperature must have been at a lower pressure
This lower P/T variation has been called Buchan-type metamorphism. It too is relatively common
Miyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-type
Both terms are common in the literature, and mean essentially the same thing
Voluminous Permian through Jurassic sedimentation and volcanism graywackes, tuffs, and some volcanics in a deep trough that was metamorphosed in the Cretaceous
The fine grain size and immature nature of the material makes it highly susceptible to metamorphic alteration, even at low grades
The “type locality” of burial metamorphism
The isograds mapped at the lower grades are listed below and are well represented in the Haast River section (Fig. 21-11 - next frame)
Orogenic belts typically proceed directly from diagenesis to chlorite or biotite zones
The development of low-grade zones in New Zealand may reflect the highly unstable nature of the tuffs and graywackes, and the availability of hot water, whereas pelitic sediments may not react until higher grades
Shikoku and Honshu in Japan: a pair of parallel metamorphic belts are exposed along a NE-SW axis parallel to the active subduction zone
These belts are of the same age, suggesting that they developed together
The NW belt (“inner” belt, inward, or away from the trench) is the Ryoke (or Abukuma) Belt
Low P/T Buchan-type of regional orogenic metamorphism
Dominant meta-pelitic sediments, and isograds up to the sillimanite zone have been mapped
A high-temperature-low-pressure belt, and granitic plutons are common
Outer belt, called the Sanbagawa Belt
It is of a high-pressure-low-temperature nature
Only reaches the garnet zone in the pelitic rocks
Basic rocks are more common than in the Ryoke belt, however, and in these glaucophane is developed (giving way to hornblende at higher grades)
Rocks are commonly called blueschists
Two belts are in contact along their whole length across a major fault zone (the Median Line)
Ryoke-Abukuma lithologies are similar to seds derived from a relatively mature volcanic arc
Sanbagawa lithologies more akin to the oceanward accretionary wedge where distal arc-derived sediments and volcanics mix with oceanic crust and marine sediment
Fig. 16-15 suggests that the 600oC isotherm, for example, could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arc
Miyashiro (1961, 1973) noted the paired nature of the Ryoke-Sanbagawa belts, and suggested …
Coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common
Called these paired metamorphic belts
May be separated by 100-200 km of less metamorphosed and deformed material (“arc-trench gap”) or closely juxtaposed (Ryoke-Sanbagawa)
In the latter cases the contact is commonly a major fault
Most of these belts are quite complex, and are not always coeval
First effects (1-2 km from contact) = 0.2 - 2.0 mm sized black ovoid “spots” in the slates
At the same time, recrystallization -> slight coarsening of the grains and degradation of the slaty cleavage
Spots were probably cordierite or andalusite, since re-hydrated and retrograded back to fine aggregates of mostly muscovite
Both cordierite and andalusite occur at higher grades, where they are often partly retrograded, but not farther out
Spots that we now see in most of the spotted slates are probably pseudomorphs
Cordierite forms ovoid xls with irregular outlines and numerous inclusions, in this case of biotite, muscovite, and opaques
The biotite and muscovite inclusions often retain the orientation of the slaty cleavage outside the cordierites
This indicates that the growing cordierite crystals enveloped aligned micas that grew during the regional event
Excellent textural evidence for the overprint of contact metamorphism on an earlier regional one
Micas outside the cordierites are larger and more randomly oriented, suggesting that they formed or recrystallized during the later thermal event
Andalusites have fewer inclusions than cordierite, and many show the cruciform pattern of fine opaque inclusions known as chiastolite
Both andalusite and cordierite are minerals characteristic of low-pressure metamorphism, which is certainly the case in the Skiddaw aureole, where heat is carried up into the shallow crust by the granites
The rocks of the inner zone at Skiddaw are characterized by coarser and more thoroughly recrystallized textures
Same mineral assemblage as the middle zone
Some rocks are schistose, but in the innermost portions the rock fabric loses the foliation, and the rocks are typical hornfelses
The zones determined on a textural basis
A more modern approach conform to the regional example above, and use sequential appearance of minerals and isograds to define the zones
First new mineral in most slates is biotite, followed by the approximately simultaneous development of cordierite and andalusite
Perhaps the textural zonation is more useful in some cases
Comrie: hotter-
Orthopyroxene occurs in pelitic and quartzo-feldspathic rocks only at the very highest grades of contact and regional metamorphism, grades that may not be reached prior to melting in many instances
Typical mineral assemblages = hypersthene + cordierite + orthoclase + biotite + opaques
Some very interesting silica-undersaturated rocks also occur in the inner aureole
Contain such non-silicate high-temperature phases as corundum and Fe-Mg spinel
Tilley noted that the low-silica rocks occur only in the inner aureole, and attributed their origin to loss of SiO2 into the diorite
Better explanation is that SiO2 (and H2O) were scavenged by granitic partial melts formed in the sediments adjacent to the contact with the hot diorite
In this progression we can see the sequential development of index minerals, such as clinohumite, followed by forsterite, clintonite, monticellite, melilite, spurrite/tillyite, merwinite, vesuvianite, diopside, wollastonite, and finally grossular garnet
The list of zones is at first quite confusing, and again serves to illustrate a common problem faced by petrologists (and probably all scientists)
We can collect quality data, but can become overwhelmed by the quantity at times, and it is often difficult to recognize meaningful patterns
Two approaches are helpful in this case:
If this is so, one ought to be able to relate minerals in the lower and next higher zone by a balanced chemical reaction
For example, the step from the first zone to the second zone is (a)
When we address isograds as reactions we can then turn to what variables are involved
In the present case, we discover than the majority of these prograde reactions consume SiO2
Since quartz is not found in the Crestmore aureole, the SiO2 must be added in the form or dissolved silica in hydrothermal fluids
We can thus conclude that diffusion of silica from the monzonite into the country rocks must play a critical role in the aureole development
Addressing the list of mineral assemblages in the zones at Crestmore can be bewildering
Find a way to display…
What if we graphically plot the minerals on a triangular CaO-MgO-SiO2 diagram?
Silica-saturated water escaping from the porphry permeates the silica-free marbles and a gradient in silica content results due to the diffusion
Silica reacts with the carbonates to produce skarns consisting of Ca-Mg silicates, while CO2 is liberated by the reactions
The porphyritic nature of the pluton thus supports the idea of fluid release
The zones at Crestmore could have formed at constant temperature, and reflect a diffusion gradient in SiO2 only
This is probably not the case, however, since temperature should also increase toward the pluton
Only by knowing the pressure-temperature stability ranges of the minerals, and the pressure-temperature dependence of the reactions relating them, could we fully understand the processes at Crestmore