Fresh basalt and weathered basalt

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.”

• Processes are indistinguishable
• Metamorphism begins in the range of 100-150 o C for the more unstable types of protolith
• Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary

• High-temperature limit grades into melting
•
Over the melting range solids and liquids coexist
•
Xenoliths, restites, and other enclaves?
• Migmatites (“mixed rocks”) are gradational

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.

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

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

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.

Metamorphic Agents and Changes
•

Metamorphic Agents and Changes
•
Lithostatic pressure - uniform stress (hydrostatic)
•
Deviatoric stress = pressure unequal in different directions
•
Resolved into three mutually perpendicular stress
( s
) components: s
1 s
2 s
3 is the maximum principal stress is an intermediate principal stress is the minimum principal stress
•
In hydrostatic situations all three are equal

• Stress
• Strain  deformation
•
Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage
•
Strain energy may overcome kinetic barriers to reactions

•
Foliation is a common result, which allows us to estimate the orientation of s
1 s
1
Strain ellipsoid
 s
1
 s
1
 s
1
> s
2
= s
2
> s
2
= s
3
 foliation and no lineation
> s
3
 lineation and no foliation
> s
3
 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.

Metamorphic Agents and Changes
Shear motion occurs along planes at an angle to s
1 s
1
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.

Metamorphic Agents and Changes
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

Metamorphic Agents and Changes
•
fluid
S
fluid
p
H2O
p
CO2
•
H2O
CO2
• p
H2O
H2O x
fluid
•
fluid
• 

The Types of Metamorphism
Different approaches to classification
1. Based on principal process or agent
• Dynamic Metamorphism
•
• Thermal Metamorphism
Dynamo-thermal Metamorphism

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

The Types of Metamorphism
Contact Metamorphism
The size and shape of an aureole is controlled by:
•
The nature of the pluton



Size
Shape
Orientation


Temperature
Composition
•
The nature of the country rocks



Composition
Depth and metamorphic grade prior to intrusion
Permeability

•
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

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

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)

The Types of Metamorphism
Very high temperatures at low pressures, generated by a volcanic or sub-volcanic body
Also developed in xenoliths

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

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

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.

The Types of Metamorphism
Orogenic Metamorphism

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.

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

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.

The Types of Metamorphism
Hydrothermal metamorphism
•
Hot H
2
O-rich fluids
•
Usually involves metasomatism
•
Difficult type to constrain: hydrothermal effects often play some role in most of the other types of metamorphism

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?

The Types of Metamorphism
Burial metamorphism occurs in areas that have not experienced significant deformation or orogeny
•
Bengal Fan
 sedimentary pile > 22 km
•
Extrap.

250-300 o C at the base (P ~ 0.6 GPa)
•
Passive margins often become active
•
Areas of burial metamorphism may thus become areas of orogenic metamorphism

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

The Types of Metamorphism
Fault-Zone and Impact Metamorphism

High rates of deformation and strain with only minor recrystallization

Impact metamorphism at meteorite (or other bolide) impact craters

Both correlate with dynamic metamorphism , based on process

(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.

•
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

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

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

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, CO
2
5. Quartz nearly pure SiO
2
.
6. Quartzo-feldspathic high Si, Na, K, Al

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

Orogenic Regional Metamorphism of the Scottish Highlands
•
George Barrow (1893, 1912)
•
SE Highlands of Scotland - Caledonian Orogeny
~ 500 Ma
•
Nappes
•
Granites

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.

•
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

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 Al
2
SiO
5
)

• 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)

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.

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

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

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 Al
2
SiO
5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al
2
SiO
5 to pyrophyllite, which limits the occurrence of an Al
2
SiO
5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

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)

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.

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)

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.

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.

Paired Metamorphic Belts of Japan

Figure 21.13.
Some of the paired metamorphic belts in the circum-Pacific region. From Miyashiro
(1994) Metamorphic
Petrology. Oxford
University Press.

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

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK
•
The aureole around the Skiddaw granite was subdivided into three zones, principally on the basis of textures:
• Unaltered slates
Increasing
Metamorphic
Grade
•
• Outer zone of spotted slates
Middle zone of andalusite slates
•
• Inner zone of hornfels
Skiddaw granite
Contact

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.

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
. Cordieriteandalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978)
Petrology of the
Metamorphic Rocks . George
Allen & Unwin. London.
1 mm

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK
Inner zone:
Thoroughly recrystallized
Lose foliation
1 mm
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.

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

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):

•
Forsterite Zone:
 calcite + brucite + clinohumite + spinel
 calcite + clinohumite + forsterite + spinel
 calcite + forsterite + spinel + clintonite
•
Monticellite Zone:
 calcite + forsterite + monticellite + clintonite
 calcite + monticellite + melilite + clintonite

 calcite + monticellite + spurrite (or tilleyite) + clintonite
 monticellite + spurrite + merwinite + melilite
•
Vesuvianite Zone: vesuvianite + monticellite + spurrite + merwinite + melilite
 vesuvianite + monticellite + diopside + wollastonite
•
Garnet Zone:
 grossular + diopside + wollastonite

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.

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

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 + SiO
2

9 Forsterite + 2 H
2
O b) Formation of the vesuvianite zone involves the reaction:
Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite
+ 15 SiO
2
+ 12 H
2
O

6 Vesuvianite + 2 CO
2

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-SiO
2
-CO
2
-H
2
O (with minor Al
2
O
3
)

Figure 21.18. CaO-MgO-SiO
2 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.
Zones are numbered
(from outside inward)

Figure 21.4.
A situation in which lithostatic pressure (P lith
) exerted by the mineral grains is greater than the intergranular fluid pressure (P fluid
). At a depth around 10 km
(or T around 300 o C) 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 P fluid until it equals P lith
. Winter (2001)
An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.

Figure 21.5.
Temperature distribution within a 1-km thick vertical dike and in the country rocks (initially at 0 o C) as a function of time. Curves are labeled in years. The model assumes an initial intrusion temperature of 1200 o C 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.