14.2 Metamorphic Minerals and Textures

Metamorphic minerals and textures are key to understanding how rocks change under pressure and heat. They reveal the story of a rock's journey through Earth's crust, showing us the conditions it faced and how it transformed.

These minerals and textures are like nature's fingerprints, each unique to specific environments. By studying them, we can piece together the puzzle of Earth's dynamic processes, from mountain building to plate tectonics.

Metamorphic Minerals and Facies

Common Metamorphic Minerals and Their Formation

Top images from around the web for Common Metamorphic Minerals and Their Formation

• 

  Overview of Metamorphic Rocks – Introductory Physical Geology Laboratory Manual – First Canadian ... View original

  Is this image relevant?

• 

  Overview of Metamorphic Rocks – Laboratory Manual for Earth Science View original

  Is this image relevant?

• 

  Putting It Together: Rocks and the Rock Cycle | Geology View original

  Is this image relevant?

• 

  Overview of Metamorphic Rocks – Introductory Physical Geology Laboratory Manual – First Canadian ... View original

  Is this image relevant?

• 

  Overview of Metamorphic Rocks – Laboratory Manual for Earth Science View original

  Is this image relevant?

1 of 3

Top images from around the web for Common Metamorphic Minerals and Their Formation

• 

  Overview of Metamorphic Rocks – Introductory Physical Geology Laboratory Manual – First Canadian ... View original

  Is this image relevant?

• 

  Overview of Metamorphic Rocks – Laboratory Manual for Earth Science View original

  Is this image relevant?

• 

  Putting It Together: Rocks and the Rock Cycle | Geology View original

  Is this image relevant?

• 

  Overview of Metamorphic Rocks – Introductory Physical Geology Laboratory Manual – First Canadian ... View original

  Is this image relevant?

• 

  Overview of Metamorphic Rocks – Laboratory Manual for Earth Science View original

  Is this image relevant?

1 of 3

• Metamorphic minerals form through recrystallization or alteration of pre-existing minerals under changing pressure and temperature conditions
• Key index minerals for metamorphism indicate specific pressure-temperature conditions
  • Garnet forms under moderate to high pressure and temperature
  • Staurolite typically indicates medium-grade metamorphism
  • Kyanite forms under high pressure conditions
  • Sillimanite indicates high temperature, moderate pressure conditions
  • Andalusite forms under low pressure, moderate temperature conditions
• Hydrous minerals (chlorite, muscovite) dominate in low-grade metamorphism
• Anhydrous minerals (garnet, pyroxene) characterize high-grade metamorphism

Metamorphic Facies and Their Characteristic Minerals

• Greenschist facies occurs at low to moderate temperatures and pressures
  • Characterized by chlorite, actinolite, and epidote
  • Common in regional metamorphism of mafic rocks (basalts)
• Amphibolite facies indicates higher temperature conditions than greenschist facies
  • Marked by abundance of hornblende and plagioclase
  • Typically forms at depths of 15-30 km in the crust
• Granulite facies represents high-grade metamorphism
  • Identified by pyroxenes, garnet, and high-temperature feldspars
  • Forms at temperatures above 700°C and moderate to high pressures
• Blueschist facies forms under high-pressure, low-temperature conditions
  • Characterized by glaucophane and lawsonite
  • Associated with subduction zones (cold, deep environments)
• Eclogite facies represents the highest pressure conditions
  • Identified by coexistence of omphacite (sodium-rich clinopyroxene) and pyrope-rich garnet
  • Occurs in subduction zones at depths greater than 35 km

Metamorphic Grade and Mineral Assemblages

Concept of Metamorphic Grade

• Metamorphic grade reflects intensity of metamorphism, typically increasing with temperature and pressure
• Prograde metamorphism involves increasing metamorphic grade
  • Leads to breakdown of low-temperature minerals
  • Forms new, higher-temperature mineral assemblages
  • Example: muscovite + quartz breaking down to form K-feldspar + sillimanite
• Retrograde metamorphism occurs during cooling and decompression
  • Can alter high-grade minerals back to lower-grade assemblages
  • Example: garnet reacting with water to form chlorite and biotite
• Metamorphic zones defined by appearance of specific index minerals
  • Represent increasing metamorphic grade in a region
  • Example: Barrovian metamorphic zones (chlorite, biotite, garnet, staurolite, kyanite, sillimanite)

Influence of Metamorphic Grade on Mineral Assemblages

• Stability fields of metamorphic minerals change with grade
  • Leads to characteristic mineral assemblages reflecting specific P-T conditions
  • Example: staurolite + quartz breaking down to form garnet + sillimanite at higher grades
• Metamorphic facies series demonstrate systematic changes in mineral assemblages
  • Barrovian sequence shows progression from low to high grade in pelitic rocks
  • Franciscan sequence represents high-pressure, low-temperature metamorphism in subduction zones
• Water content decreases with increasing metamorphic grade
  • Low-grade rocks contain hydrous minerals (chlorite, micas)
  • High-grade rocks dominated by anhydrous minerals (garnet, pyroxenes)
• Mineral compositions become more complex at higher grades
  • Example: plagioclase becomes more calcic with increasing grade in mafic rocks

Formation of Metamorphic Textures

Development of Foliation and Lineation

• Metamorphic textures result from recrystallization and deformation under stress and elevated temperatures
• Foliation forms as planar fabric in metamorphic rocks
  • Caused by alignment of platy or elongate minerals perpendicular to maximum stress direction
  • Examples include slaty cleavage, schistosity, and gneissic banding
• Lineation creates linear fabric in metamorphic rocks
  • Characterized by parallel alignment of elongate minerals or mineral aggregates
  • Can form through mineral growth, stretching, or intersection of planar features
• Schistosity characterized by parallel alignment of platy minerals (micas)
  • Gives rock a shiny, undulating appearance
  • Common in medium-grade metamorphic rocks (schists)
• Gneissic banding forms coarse foliation under high-grade conditions
  • Alternating bands of light (felsic) and dark (mafic) minerals
  • Typically seen in high-grade metamorphic rocks (gneisses)

Specialized Metamorphic Textures

• Crenulation cleavage results from microfolding of earlier foliation
  • Often seen in multiply deformed metamorphic rocks
  • Creates a wavy or corrugated appearance on foliation surfaces
• Porphyroblasts grow as large crystals during metamorphism
  • Preserve evidence of rock's deformation history in internal structures
  • Examples include garnet, staurolite, and andalusite crystals
• Reaction rims form around pre-existing minerals during metamorphism
  • Indicate incomplete reactions or changing metamorphic conditions
  • Example: corona textures in mafic granulites
• Mylonitic textures develop in shear zones under high strain rates
  • Characterized by extreme grain size reduction and flow structures
  • Can form in both igneous and metamorphic protoliths

Metamorphic Minerals vs Pressure-Temperature Conditions

Interpreting Mineral Assemblages and P-T Conditions

• Metamorphic mineral assemblages indicate P-T conditions of formation
  • Allow reconstruction of rock's metamorphic history
  • Example: kyanite + staurolite assemblage indicates medium to high pressure, moderate temperature
• Pressure-temperature (P-T) diagrams illustrate stability fields of mineral assemblages
  • Used to determine conditions of metamorphism
  • Show reaction boundaries between different mineral assemblages
• Geothermobarometry estimates P-T conditions quantitatively
  • Uses chemical compositions of coexisting minerals
  • Examples include garnet-biotite thermometry and garnet-plagioclase-quartz-aluminum silicate barometry
• Metamorphic reactions occur at specific P-T conditions
  • Act as indicators of metamorphic grade
  • Example: staurolite breakdown to form kyanite and garnet at higher pressures

Specialized Indicators of Metamorphic Conditions

• Polymorphs have different stability fields constraining P-T conditions
  • Al2SiO5 minerals (andalusite, kyanite, sillimanite) commonly used
  • Triple point of Al2SiO5 system occurs at approximately 500°C and 4 kbar
• Certain minerals indicate specific metamorphic environments
  • Lawsonite or jadeite suggest high-pressure, low-temperature conditions
  • Associated with subduction zone metamorphism
• Mineral zoning in metamorphic rocks provides information on changing P-T conditions
  • Garnet zoning preserves growth history during metamorphism
  • Core-to-rim compositional changes reflect evolving P-T path
• Fluid inclusions in metamorphic minerals trap evidence of metamorphic fluids
  • Can be used to determine temperature and pressure of entrapment
  • Provide insights into fluid composition during metamorphism