Podcast
Questions and Answers
What is one type of deviatoric stress and its possible resulting structure?
What is one type of deviatoric stress and its possible resulting structure?
Which types of fluids are recognized as metamorphic fluids?
Which types of fluids are recognized as metamorphic fluids?
What is required for the growth of hydrous or carbonate phases?
What is required for the growth of hydrous or carbonate phases?
What structure can be formed as a result of metasomatism?
What structure can be formed as a result of metasomatism?
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Which classification is based on the principal process or agent in metamorphism?
Which classification is based on the principal process or agent in metamorphism?
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What is the primary factor in metamorphism?
What is the primary factor in metamorphism?
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What effect does increasing temperature generally have on grain size during metamorphism?
What effect does increasing temperature generally have on grain size during metamorphism?
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In what type of geological setting would one expect a low temperature-high pressure geotherm?
In what type of geological setting would one expect a low temperature-high pressure geotherm?
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Which of the following statements about metamorphic field gradients is true?
Which of the following statements about metamorphic field gradients is true?
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How do high temperature conditions affect the stability of minerals during metamorphism?
How do high temperature conditions affect the stability of minerals during metamorphism?
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What primarily determines the size and shape of an aureole in contact metamorphism?
What primarily determines the size and shape of an aureole in contact metamorphism?
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Which type of metamorphism is most likely to occur adjacent to igneous intrusions?
Which type of metamorphism is most likely to occur adjacent to igneous intrusions?
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What type of metamorphic rock commonly develops during contact metamorphism?
What type of metamorphic rock commonly develops during contact metamorphism?
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Which of the following is NOT a factor that influences contact metamorphism?
Which of the following is NOT a factor that influences contact metamorphism?
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Polymetamorphic rocks are indicative of which geological process?
Polymetamorphic rocks are indicative of which geological process?
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What event is associated with the formation of spotted phyllite?
What event is associated with the formation of spotted phyllite?
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What causes the formation of black ovoid spots in slates during contact metamorphism?
What causes the formation of black ovoid spots in slates during contact metamorphism?
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In the Skiddaw aureole, which mineral is commonly found in the middle zone of slates?
In the Skiddaw aureole, which mineral is commonly found in the middle zone of slates?
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Which zone in the Skiddaw aureole is characterized by unaltered slates?
Which zone in the Skiddaw aureole is characterized by unaltered slates?
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What is the primary factor for the alteration of Skiddaw slates to spotted slates?
What is the primary factor for the alteration of Skiddaw slates to spotted slates?
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What are the sizes of the black ovoid spots observed in the slates during initial contact metamorphism?
What are the sizes of the black ovoid spots observed in the slates during initial contact metamorphism?
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What type of metamorphism is indicated by the presence of andalusite in slates?
What type of metamorphism is indicated by the presence of andalusite in slates?
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Which mineral is likely to retrograde back to fine aggregates after contact metamorphism?
Which mineral is likely to retrograde back to fine aggregates after contact metamorphism?
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Which minerals are characteristic of low-pressure metamorphism?
Which minerals are characteristic of low-pressure metamorphism?
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What is a key feature of the inner zone rocks in the Skiddaw aureole?
What is a key feature of the inner zone rocks in the Skiddaw aureole?
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Which mineral typically appears first in the metamorphism of slates?
Which mineral typically appears first in the metamorphism of slates?
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Pyrometamorphism is primarily characterized by what conditions?
Pyrometamorphism is primarily characterized by what conditions?
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Which type of metamorphism covers a large body of rock with great lateral extent?
Which type of metamorphism covers a large body of rock with great lateral extent?
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Orogenic metamorphism is commonly associated with which geological feature?
Orogenic metamorphism is commonly associated with which geological feature?
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In the context of metamorphism, the term 'dynamo-thermal' refers to what?
In the context of metamorphism, the term 'dynamo-thermal' refers to what?
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Which of the following is a characteristic product of orogenic metamorphism?
Which of the following is a characteristic product of orogenic metamorphism?
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Which process is typically involved in the underthrusting at convergent plate margins?
Which process is typically involved in the underthrusting at convergent plate margins?
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Which environment is typical for orogenic metamorphism?
Which environment is typical for orogenic metamorphism?
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Study Notes
Metamorphic Agents and Changes
- Temperature is the most important factor in metamorphism.
- Geothermal gradient for oceanic lithosphere is higher than for continental shields.
- Increasing temperature promotes recrystallization, increasing grain size.
- Temperature drives chemical reactions that consume unstable minerals and produce new, stable ones.
- Temperature overcomes kinetic barriers, allowing equilibrium to occur.
- Pressure typically increases with temperature.
- Most disturbances are transient, returning to normal gradients.
- High T/P geotherms occur in areas with plutonic activity or rifting.
- Low T/P geotherms occur in subduction zones.
- Metamorphic field gradients are different from geotherms, showing the typical rise in temperature with pressure change for a specific metamorphic scenario.
Fluids
- Evidence of metamorphic fluids includes fluid inclusions and requirement for hydrous or carbonate phases.
- Volatile-involving reactions occur at temperatures and pressures requiring finite fluid pressures.
- Metamorphic fluids can originate from meteoric water, magmatic sources, subducted material dehydration, sedimentary brines, or mantle degassing.
- Fluid motion can transport chemicals over long distances, leading to metasomatism.
- Fluid pressure is the sum of partial pressures of its components.
- Mole fractions of components sum to 1.
- Gradients in temperature, pressure, and fluid composition lead to zonation in mineral assemblages.
Types of Metamorphism
- Metamorphism classification can be based on process or setting.
- Dynamic metamorphism: caused by stress.
- Thermal metamorphism: caused by heat.
- Dynamo-thermal metamorphism: caused by both stress and heat.
- Contact metamorphism: occurs near igneous intrusions, resulting in thermal effects and metasomatism.
- Pyrometamorphism: a minor type of contact metamorphism caused by very high temperatures at low pressures, often associated with volcanic activity or xenoliths.
- Regional metamorphism: affects large rock bodies and includes orogenic, burial, and ocean-floor metamorphism.
- Orogenic metamorphism: associated with convergent plate margins, characterized by high geothermal gradients, deformation, and foliated rocks.
- Burial metamorphism: occurs at depth due to increasing pressure and temperature, primarily affecting sedimentary rocks.
- Ocean-floor metamorphism: occurs at mid-ocean ridges, where seawater reacts with hot rocks, leading to alteration and hydrothermal vents.
- Hydrothermal metamorphism: caused by hot, chemically active fluids circulating through rocks.
- Fault-zone metamorphism: occurs along fault zones, producing localized zones of shearing and recrystallization, often characterized by cataclastic textures.
- Impact or shock metamorphism: caused by high-energy impacts, leading to extreme pressures and temperatures, resulting in unique mineral assemblages and textures.
Contact Metamorphism
- Aureole size and shape are controlled by the intruding pluton (size, shape, temperature, composition, orientation) and the country rocks (composition, depth, pre-existing metamorphic grade, permeability).
- Depending on the protolith, different metamorphic rock types will develop.
- Commonly occurs near igneous intrusions, involving thermal and metasomatic effects of hot magma intruding cooler, shallow rocks.
- Occurs across a wide range of pressures, including very low pressures.
- A contact aureole is a zone of metamorphic rocks surrounding the intrusion.
Contact Metamorphism: Hornfelses
- Most recognizable when a pluton intrudes shallow rocks in a static environment.
- Hornfelses (granofelses) result from contact metamorphism, often with retained textures and structures.
Contact Metamorphism: Polymetamorphism
- Represents an orogenic event followed by a contact metamorphism.
- Spotted phyllite (or slate) is an example, where the foliated phyllite formed during regional event, and the ovoid spots are minerals that grew during the contact event.
Contact Metamorphism: The Skiddaw Aureole, UK
- Ordovician Skiddaw Slates (English Lake District) intruded by granitic bodies.
- Intrusions are shallow, contact effects overprinted an earlier low-grade regional orogenic metamorphism.
- Skiddaw aureole is subdivided into three zones based on textures:
- Unaltered slates.
- Outer zone of spotted slates.
- Middle zone of andalusite slates.
- Inner zone of hornfels.
Skiddaw Aureole Zones
- First effects occur 1-2 km from the contact, characterized by 0.2-2.0 mm black ovoid "spots" in the slates, slight grain coarsening, and slaty cleavage degradation.
- Spots are likely cordierite or andalusite, subsequently re-hydrated and retrograded to fine muscovite aggregates.
- Cordierite and andalusite are high-grade minerals, often partly retrograded.
- Spots in most spotted slates are likely pseudomorphs.
- Middle zone: slates are more thoroughly recrystallized, containing biotite, muscovite, cordierite, andalusite, and quartz.
- Inner zone: thoroughly recrystallized, contains andalusite and cordierite, indicating low-pressure metamorphism, with coarser and more thoroughly recrystallized textures.
- Inner zone rocks are schistose, losing foliation in innermost portions, becoming typical hornfelses.
Skiddaw Aureole: Zone Definition
- Zones are determined on a textural basis.
- It's preferable to define zones using sequential mineral appearance and isograds rather than textures.
- The first new mineral in most slates is biotite, followed by the simultaneous development of cordierite and andalusite.
- Orthopyroxene occurs in pelitic and quartzo-feldspathic rocks at the highest grades of contact and regional metamorphism.
- Orthopyroxene may not be reached before melting in many instances.
Pyrometamorphism
- A minor type of contact metamorphism caused by very high temperatures at low pressures from volcanic or sub-volcanic bodies.
- Also occurs in xenoliths.
- May be accompanied by various degrees of partial melting.
Regional Metamorphism
- Metamorphism that affects a large rock body.
- Three main types:
- Orogenic metamorphism.
- Burial metamorphism.
- Ocean-floor metamorphism.
Orogenic Metamorphism
- Associated with convergent plate margins.
- Dynamo-thermal: characterized by combined elevated geothermal gradients and deformation (deviatoric stress).
- Foliated rocks are characteristic.
- Typical environments: island arcs, active continental margins, and continental collision zones.
- Often used synonymously with "regional metamorphism".
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Description
Test your knowledge on the key factors influencing metamorphism, particularly the role of temperature and pressure. Explore how these elements drive geological processes and the formation of metamorphic rocks. This quiz covers essential concepts of metamorphic geology and fluid involvement.
