Introduction to Metamorphism IN THIS LECTURE –Definition of Metamorphism –Limits of Metamorphism –Agents of Metamorphic Change o Temperature o Pressure

Introduction to Metamorphism

IN THIS LECTURE

– Definition of Metamorphism– Limits of Metamorphism– Agents of Metamorphic Change

o Temperatureo Pressureo Stress and Straino Fluids

– Metamorphic Change– Classification of Metamorphic Rocks– Types of 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:

1. Melt2. Cpx + plag ( olivine, ilmenite…)3. Or any combination of melt + minerals along the liquid

line of descent

• If uplifted and eroded surface, will weather a combinations of clays, oxides…

• 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.”


The Limits of Metamorphism

• Low-temperature limit grades into diagenesis• The boundary is somewhat arbitrary

o Diagenetic/weathering processes are indistinguishable from metamorphic

o Metamorphism begins in the range of 100-150oC for the more unstable types of protolith

o Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary


• High-temperature limit grades into melting

o Over the melting range solids and liquids coexist

o If we heat a metamorphic rock until it melts, at what point in the melting process does it become “igneous”?

o Xenoliths, restites, and other enclaves are considered part of the igneous realm because melt is dominant, but the distinction is certainly vague and disputable

o Migmatites (“mixed rocks”) are gradational

• Migmatites: metamorphic or igneous rocks?


Limits of Metamorphism

Agents of Metamorphic Change

• There are several main agents of metamorphic change

1. Temperature2. Pressure3. Stress and Strain4. Fluids

Agents of Metamorphic Change1. 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, 269-311.

Agents of Metamorphic Change1. Temperature

Increasing temperature has several effects

1. 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

2. Drive 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

Agents of Metamorphic Change2. Pressure

• PRESSURE– Normal gradients may be perturbed in several ways

– The two main examples are

o High T/P geotherms in areas of plutonic activity or rifting

o Low T/P geotherms in subduction zones

Agents of Metamorphic Change3. 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

Agents of Metamorphic Change4. 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)

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 usedo Low Gradeo Medium Gradeo High Grade

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

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

Classification of Metamorphic Rocks

• Two main ways in which metamorphic rocks are classified

1. Based on the style or type of metamorphism2. Based on the mineral assemblage or texture of the

rocks

– We’ll look first at the types of metamorphism

The Types of Metamorphism

Different approaches to classification based on type

1. Based on principal process or agento Dynamic Metamorphismo Thermal Metamorphismo Dynamo-thermal Metamorphism

The Types of Metamorphism

2. Based on settingo Contact Metamorphism

• Pyrometamorphism

o Regional Metamorphism• Orogenic Metamorphism• Burial Metamorphism• Ocean Floor Metamorphism

o Hydrothermal Metamorphism

o Fault-Zone Metamorphism

o Impact or Shock Metamorphism

• We’ll look at the classifivation based on setting

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


• The size and shape of an aureole is controlled by– The nature of the pluton, ie

o Sizeo Shapeo Orientationo Compositiono Temperature

– The nature of the country rockso Compositiono Depth and metamorphic grade prior to intrusiono Permeability


– 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: o Lag time between the creation of the magma at

depth during T maximum, and its migration to the lower grade rocks above

o Plutonism may reflect a separate phase of post-orogenic collapse magmatism


Pyrometamorphismo Very high temperatures at very low pressures,

generated by a volcanic or subvolcanic bodyo Also developed in xenolithso Not very common and won’t be looked at further in

this course

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


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


OROGENIC METAMORPHIS

M

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.


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


OROGENIC METAMORPHISM

• Batholiths are usually present in the highest grade areas

• If plentiful and closely spaced, may be called regional contact 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.


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


BURIAL METAMORPHISM• Bengal Fan represents a sedimentary pile > 22

km• Extrapolating implies 250-300oC 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


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!


OCEAN-FLOOR METAMORPHISM• May be considered another example of hydrothermal

metamorphism• Highly altered chlorite-quartz rocks- distinctive high-Mg,

low-Ca composition

Documents

Introduction to Metamorphism IN THIS LECTURE –Definition of Metamorphism –Limits of Metamorphism –Agents of Metamorphic Change o Temperature o Pressure