7.3 Metamorphic rock classification and facies

Metamorphic rocks tell a story of transformation under intense heat and pressure. They're classified by texture and mineral composition, reflecting the conditions of their formation. From slate to gneiss, each rock type reveals clues about its journey through Earth's crust.

Metamorphic facies provide a framework for understanding these rocks' origins. By mapping mineral assemblages to specific pressure-temperature conditions, geologists can reconstruct the tectonic history of an area. This knowledge helps piece together Earth's dynamic past and present.

Metamorphic Rock Classification

Classification of metamorphic rocks

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• Texture classification determines rock structure and appearance
  • Foliated rocks display aligned minerals or layering
    • Slaty cleavage shows fine, planar splitting (slate)
    • Schistose texture exhibits medium-grained platy minerals (schist)
    • Gneissic texture displays coarse mineral banding (gneiss)
  • Non-foliated rocks lack preferred mineral orientation
    • Granoblastic texture shows interlocking equidimensional grains (marble)
    • Hornfelsic texture exhibits fine-grained, compact appearance (hornfels)
• Mineral composition classification reflects rock chemistry and formation conditions
  • Common metamorphic minerals form under specific P-T conditions
    • Quartz persists through various grades of metamorphism
    • Feldspars include plagioclase and K-feldspar, common in higher grades
    • Micas (biotite, muscovite) indicate moderate metamorphic conditions
    • Amphiboles (hornblende) signify medium to high-grade metamorphism
    • Garnet forms under high pressure or temperature conditions
  • Protolith influence on mineral assemblage determines final rock composition
• Metamorphic grade indicates intensity of metamorphism
  • Low-grade metamorphism occurs at relatively low P-T conditions (greenschist facies)
  • Medium-grade metamorphism represents intermediate P-T conditions (amphibolite facies)
  • High-grade metamorphism occurs at extreme P-T conditions (granulite facies)

Types of common metamorphic rocks

• Slate forms from low-grade metamorphism of shale or mudstone
  • Fine-grained, foliated texture with slaty cleavage
  • Often used for roofing tiles or chalkboards
• Schist develops from medium-grade metamorphism of various protoliths
  • Medium-grained, foliated texture with abundant platy minerals (micas)
  • Schistose texture allows rock to split along mica-rich layers
• Gneiss results from high-grade metamorphism of diverse protoliths
  • Coarse-grained, foliated texture with alternating light and dark bands
  • Gneissic texture shows distinct segregation of felsic and mafic minerals
• Marble forms from metamorphism of limestone or dolostone
  • Non-foliated texture with interlocking calcite or dolomite crystals
  • Often used in sculpture and architecture (Taj Mahal)

Metamorphic Facies and Conditions

Concept of metamorphic facies

• Metamorphic facies represent specific P-T conditions producing characteristic mineral assemblages
• Key metamorphic facies correspond to different tectonic settings
  • Zeolite facies forms at very low-grade conditions (burial metamorphism)
  • Greenschist facies develops in low to medium-grade regional metamorphism
  • Amphibolite facies occurs in medium to high-grade regional metamorphism
  • Granulite facies forms at high-grade conditions (deep continental crust)
  • Blueschist facies develops in high-pressure, low-temperature environments (subduction zones)
  • Eclogite facies occurs at extreme high-pressure conditions (deep subduction)
• Pressure-temperature conditions for each facies determine mineral stability
• Index minerals indicate metamorphic grade within specific facies (chlorite, garnet, kyanite)

Interpretation of facies diagrams

• Components of a metamorphic facies diagram illustrate P-T relationships
  • Pressure axis typically ranges from 0 to 20 kbar
  • Temperature axis usually spans 0 to 1000℃
  • Facies boundaries separate stability fields of different mineral assemblages
• Stability fields of important minerals show P-T ranges for mineral formation
  • Chlorite stable at low-grade conditions
  • Biotite appears at medium-grade conditions
  • Garnet forms under increasing pressure or temperature
  • Aluminosilicates (kyanite, andalusite, sillimanite) indicate specific P-T conditions
• Reaction lines and mineral assemblages represent changes in mineral stability
• Facies diagrams help determine metamorphic conditions of rock formation

Metamorphism in plate tectonics

• Metamorphic grade progression reflects changing P-T conditions
  • Low to high grade transitions occur with increasing depth or temperature
  • Prograde metamorphism involves increasing P-T conditions
  • Retrograde metamorphism occurs during cooling and exhumation
• Plate tectonic settings produce characteristic metamorphic facies
  • Subduction zones generate blueschist and eclogite facies
  • Continental collision creates greenschist, amphibolite, and granulite facies
  • Contact metamorphism near magma bodies produces hornfels
• Pressure-temperature-time (P-T-t) paths reveal tectonic history
  • Clockwise paths often indicate crustal thickening followed by erosion
  • Counterclockwise paths suggest rapid exhumation or heating
• Metamorphic facies series reflect different geothermal gradients
  • Barrovian series (medium pressure) typical of continental collision (Scottish Highlands)
  • Buchan series (low pressure, high temperature) associated with magmatism
  • High pressure-low temperature series found in subduction zones (Japan)