topic 8 ( metamorphoc facies series )

• Metamorphic Facies Series

  Sequence of facies occurring across a metamorphic terrane due to differences in pressure and temperature conditions related to space and time.
• Low P/T

  • Contact Facies Series
  • Buchan Facies Series
• Moderate P/T

  • Barrovian Facies Series
• High P/T

  • Sanbagawa Facies Series
  • Franciscan Facies Series
• • Contact Facies Series
  • Low pressure (<2.5 kbar), 8km depth, but moderate to high temperature; geothermal gradient is >80oC/km
  • Higher heat input due to magmatic intrusion; contact metamorphism in aureoles;
  • Recorded by Hornfels Facies
• Buchan/Abukuma Facies Series

  • Defined by low to moderate P/T assemblages; geothermal gradient of 40oC/km – 80oC/km
  • Reflect high temperature but moderate pressure
  • Develop by regional metamorphism and convergent magmatic activity; commonly foliated
• 1. Barrovian Facies Series

  • Reflect progressive increase in temperature and pressure during regional metamorphism; geothermal gradient of 20oC/km – 40oC/km
  • In thickening orogenic belts; display foliated texture
• Sanbagawa Facies Series

  • Reflect rapid pressure increase relative to temperature during regional metamorphism at convergent boundary; geothermal gradient of 10oC/km – 20oC/km
  • Slightly temperature from slower subduction or higher geothermal gradient during subduction
• 1. Franciscan Facies Series

  • Unusual high P/T ratio in progressive metamorphism; geothermal gradient of < 10oC/km
  • Reflects progressive rapid pressure increase relative to slow increase in temperature in subduction zones
  • Geobarometer markers: jadeite, glaucophane, lawsonite
• • Divergent Boundaries

  continental rifts

  • Contact metamorphism from magmatic intrusion
  • Hydrothermal alteration associated with hot magmatic and wall rock volatile fluids
  • Dynamic metamorphism due to brittle extensional faulting (upper crust) and ductile shearing (lower crust)
  • Burial metamorphism producing Zeolite Facies and Prehnite-Pumpellyite Facies due to thick deposition of non-marine detrital sediment sequences in rift basins
• B. Ocean Ridges

  • Extensive hydrothermal metamorphism resulting to Albite-Epidote Hornfels Facies, Zeolite Facies and Prehnite-Pumpellyite Facies. 
  • At deeper levels, Hornblende Facies are produced. 
• • Convergent Boundaries

  a . Outer Metamorphic Belts

  • Outer metamorphic belt on ocean or trench side, consisting of Sanbagawa Facies Series (rapid steep subduction) or Franciscan Facies Series (slow shallower subduction)
  • Contain Hornfels Facies, Zeolite Facies, Prehnite-Pumpellyite Facies, overprinted by Greenschist Facies and Blueschist Facies & Eclogite Facies. 
• B. Inner Metamorphic Belt

  • Inner metamorphic belt on the continent or arc side along the magmatic arc complex, consisting of Buchan Facies Series or Barrovian Facies Series. 
• C. Ocean-Ocean convergence

     

  i. Trench and Subduction Zone

  • Ocean plate subduction results to Sanbagawa Facies Series and Franciscan Facies Series and can later be incorporated into forearc accretionary wedges and melanges. 
  • Zeolite, Prehnite-Pumpellyite, Greenschist, Amphibolite, and Granulite Facies
  • Blueschist Facies and rare Eclogite Facies
• C. Ocean-Ocean convergence

  ii. Forearc Basins and Basements

  • Between the uplifted accretionary wedge/subduction zone complex and the volcanic arc
  • Burial Metamorphism of deposited sediments, resulting to Zeolite and Prehnite-Pumpellyite Facies. 
• C. Ocean-Ocean convergence

  iii. Magmatic Arc Complexes

  • Deep intermediate to silicic plutons overlain by composite island arc volcanoes
  • Generates Contact, Buchan, and lesser Barrovian Facies Series. 
• C. Ocean-Ocean convergence

     iv. Back-arc basin and Pull-apart basin

  • Develop in response to tensional stress within arc complex
  • Zeolite, Prehnite-Pumpellyite, and Hornfels Facies at shallow depths. Greenschist Facies at deeper levels. 
  • Pull-apart basins form Sub-greenschist conditions on the Zeolite and Prehnite-Pumpellyite Facies. 
• D. Ocean-Continent Convergence

  i. Fold and Thrust Belts

  • Produce regional Buchan to Barrovian Facies Series
  • Zeolite and Prehnite-Pumpellyite Facies develop in thick deposition of sediments including flysch and molasse deposits. 
• D. Continent-Continent Convergence

  • Suture Zones marks the former subduction trench after an ocean basin has closed and subduction ceased
  • Lithospheric thickening to Barrovian Facies Series
  • Elevated P-T results to Greenschist, Amphibolite, and Granulite Facies