Metamorphic Grades and Zones | Geology Optional Notes for UPSC PDF Download

Grade of Metamorphism

• Low-grade metamorphism: This is the initial stage of metamorphism where rocks undergo slight changes due to low temperatures and pressures. An example of this is the formation of slate from shale.
• Medium-grade metamorphism: Rocks experience moderate changes under increased heat and pressure. For instance, limestone can transform into marble during medium-grade metamorphism.
• High-grade metamorphism: This advanced stage involves intense heat and pressure, leading to significant alterations in rocks. An example is the transformation of shale into schist.
• Extreme-grade metamorphism: At this stage, rocks undergo extreme changes due to very high temperatures and pressures. One example is the formation of gneiss from granite.

Metamorphic Petrology

Introduction to Metamorphism

• Metamorphism refers to the process where rocks undergo changes in mineralogy, texture, or chemical composition due to varying temperature and pressure conditions.
• Metamorphic grade is a term used to describe the intensity of metamorphism, indicating the peak temperature of recrystallization.
• Geologists categorize metamorphic grade into different levels such as 'very-low grade,' 'low grade,' 'medium grade,' 'high grade,' and 'very-high grade' based on the extent of metamorphic changes.

Metamorphic Grade Classification

• Metamorphic grade signifies the temperature and/or pressure at which a rock underwent metamorphism.
• Low-grade metamorphism occurs at temperatures ranging from 200°C to 320°C and relatively low pressure levels.
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  • Characterized by hydrous minerals like clay, mica, and chlorite.
  • Examples of low-grade metamorphic rocks include slate and phyllite.
• High-grade metamorphism happens at temperatures exceeding 320°C and higher pressure.
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  • With increasing grade, hydrous minerals decrease, while non-hydrous minerals become predominant.
  • Minerals like muscovite disappear at high grades, while biotite remains stable. Pyroxene and garnet are non-hydrous minerals.
  • High-grade metamorphic rocks include schist and gneiss.

Types of Metamorphism

• Prograde metamorphism leads to the formation of minerals typical of higher grades, indicating a progression towards higher temperatures.
• This type of metamorphism results in the development of mineral assemblages at elevated temperatures compared to the initial phase.

Unit 13: Metamorphism Summary

Prograde and Retrograde Metamorphism

• Prograde metamorphism occurs when temperature and pressure increase progressively.
• Retrograde metamorphism happens when temperature and pressure decrease, leading to minerals typical of lower grades.
• As pressure and temperature decrease, rocks may return to a lower metamorphic grade.
• Distinct grades of metamorphism result in different mineral assemblages.
• Higher grades involve recrystallization and neocrystallization, forming coarser grains and stable mineral assemblages under high temperatures and pressures.

Metamorphic Grades

• Low Grade: 200°C - 400°C
• Intermediate Grade: 400°C - 600°C
• High Grade: 600°C - 800°C
• Pressure Ranges: 200 Mpa to 800 Mpa

Zones of Metamorphism

• George Barrow and C.E. Tilley pioneered the mapping of regional metamorphic rocks in Scotland.
• They proposed dividing areas into metamorphic zones based on mineral assemblages with increasing metamorphic grade.
• Metamorphic zones are marked by the first appearance of specific index minerals.
• Index minerals indicate the temperature of formation and burial depth.
• Sequence of Index Minerals: Chlorite → Biotite → Almandine Garnet → Staurolite → Kyanite → Sillimanite

Example: Metamorphic Zones

For example, in a specific region, the presence of biotite might indicate a certain metamorphic grade, while the appearance of staurolite could signify a higher grade.

Conclusion

Understanding metamorphic grades and zones is crucial for interpreting the geological history of a region and the conditions under which rocks have formed and changed over time.

Metamorphic Petrology Summary

Introduction to Metamorphic Zones

• Metamorphic petrology involves the study of rocks that have undergone changes due to high pressure, temperature, or chemically active fluids.

Zonal Sequences in Metamorphic Petrology

• Metamorphic zones are delineated by the presence of specific index minerals that indicate varying degrees of metamorphic intensity.
• Each zone has a distinct set of minerals that are characteristic of that specific level of metamorphism.

Barrovian Zones in Metamorphic Petrology

• Barrovian zones, named after George Barrow, represent a sequence of metamorphic zones found in medium-pressure metapelites.

Specific Zones in Barrovian Sequences

• Chlorite Zone:
  • Pelitic rocks in this zone are primarily composed of chlorite, muscovite, quartz, and albite.
• Biotite Zone:
  • This zone features slates transitioning into phyllites and schists, with minerals such as biotite, chlorite, muscovite, quartz, and albite.
• Garnet Zone:
  • Schists in this zone contain prominent red almandine garnet alongside biotite, chlorite, muscovite, quartz, and albite or oligoclase.
• Staurolite Zone:
  • Schists in this zone exhibit staurolite, biotite, muscovite, quartz, garnet, and plagioclase, with possible presence of chlorite.
• Kyanite Zone:
  • Characterized by schists containing kyanite, biotite, muscovite, quartz, plagioclase, and often garnet and staurolite.
• Sillimanite Zone:
  • Includes schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and sometimes staurolite and kyanite.

Visual Representation of Metamorphic Zones

• Metamorphic zones are depicted in diagrams showing the progression of metamorphic grades through different mineral assemblages.
• These zones help geologists understand the changes rocks undergo under varying metamorphic conditions.