topic 7 ( metamorphic facies )

• Metamorphic Facies

  Distinctive mineral assemblages in metamorphic rocks formed in a specific P-T condition.

  Does not indicate genetic origin.
• Eskola (1939) -

  Sanidinite, Hornfels, Greenschist, Amphibolite, Eclogite
• Turner (1958)

  Zeolite
• Eskola (1939) & Turner (1958)

  Blueschist, Granulite
• Coombs (1961)

  Prehnite – Pumpellyite
• Eskola (1939) & 

  Fyfe et al. (1958)

  Albite-Epidote hornfels, Hornblende hornfels, Pyroxene hornfels, Sanidinite hornfels
• Hornfels Facies

  Non-foliated, fine-grained hornfels rocks and granoblastic coarse metamorphic rocks, formed by contact metamorphism.
• 1. Albite-Epidote Hornfels Facies

  Low-T facies at <450oC, <2kbar <6km depth.

  Develop on outer fringes of aureoles. 

  Low-P equivalent of Greenschist Facies.
• 1. Hornblende Hornfels Facies

  Medium-T at 450-600oC, <2kbar, and <8km depth

  Bulk of many metamorphic aureoles

  Absent chlorite, albite, epidote, actinolite

  Low-P equivalent of Amphibolite Facies
• 1. Pyroxene Hornblende Facies

  High-T at 600-800oC, <2.5kbar, <8km depth

  Commonly adjacent to higher-T basic intrusions

  Hydrous minerals do not occur, except biotite; dehydration

  Low-P equivalent of Granulite Facies
• 1. Sanidinite Hornfels Facies

  Very high-T at >800oC, <2.5kbar, <8km depth

  Associated w/ basic & ultrabasic intrusions or xenoliths

  Hydrous minerals are absent 
• Zeolite Facies

  Low-grade metamorphism at 150oC to 300oC, <5 kbar, and at 15km depth

  Zeolites are hydrous Na- & Ca- Al-tectosilicate from diagenetic or low-T metamorphic reactions.

  Laumontite and Heulandite are common.
• Zeolite Facies

  From hydrothermal alteration of volcanic protoliths, the devitrification of basaltic glass and tuff, and the reaction of pelites & graywackes with saline water.

  Occurs in isolated vesicles or disseminated within the rock, preserving relict structures.
• Prehnite – Pumpellyite Facies

  Produced by hydrothermal alteration and burial metamorphism at 250oC – 350oC, <6kbar, and 20km depth. 

  Prehnite & Pumpellyite; retain relict structures
• Greenschist Facies

  Medium P-T at 350–550oC, 3-10kbar, 10-30km depth associated with Dynamothermal metamorphism at convergent boundaries. 

  Key minerals are Epidote, Chlorite, & Actinolite; imparts green color and schistosity.

  Increasing temperature removes volatiles.
• Amphibolite Facies

  High-T and Moderate to High-P at 550-750oC, 4-12kbar, and 12-40km depth in orogenic belts

  Transition from Greenschist Facies marked by increased in hornblende, garnet & anthophyllite and decrease in actinolite, chlorite, biotite, talc.

  Plagioclase become more calcic than sodic

  Marks the appearance of Staurolite with Kyanite.
• Granulite Facies

  High-T and Moderate- to High-P at 700-900oC, 3-15kbar, 10-50km depth

  Dominated by anhydrous granoblastic textures. Granulite I has hornblende & biotite

  Granulite II are entirely anhydrous minerals

  Amphiboles dehydrate to Pyroxene and Phyllosilicates dehydrate into Anhydrous minerals
• Granulite Facies

  Quartz and Orthoclase common in pelitic and quartzofeldspathic protoliths

  Wollastonite and absence of Phlogopite common in calcareous rocks.

  Orthopyroxene and clinopyroxene common in metabasic and metaultrabasic protoliths
• Blueschist Facies

  Moderate to High-P, low-T at 150-500oC, 4-20kbar, 13-66km depth

  Unusually high P-T ratio reflecting unusually low geothermal gradient.

  Glaucophane provides the blue color;

  Mg-riebeckite, aegirine, crossite, kyanite, lawsonite, jadeite
• Blueschist Facies

  In subduction zones where oceanic lithosphere is subducted at rapid rates. Subduction of cold lithosphere lowers the temperature of subduction, resulting to depression of isotherms, creating a set of high-P low-T assemblages. 

  Critical indicator of subduction zones in convergent margins.
• Eclogite Facies

  High-T & very high-P at 400-900oC, 12-25kbar, 40-82km depth

  Chemically similar to silica-undersaturated anhydrous basalt

  Fine- to coarse-grained dark green rocks with reddish-brown porphyroblasts.

  Papua New Guinea has the youngest eclogites
• UHP Facies

  Ultra-high pressure at >27kbar, 800-1200oC, >80km depth

  Related to Eclogite; Facies where Coesite is stable

  Recognized by:

  • Presence of Coesite, Diamond, or Majoritic garnet
  • Magnesite + Aragonite
  • Garnet + Cpx + K-white mica
  • Garnet + Cpx + Kyanite + Coesite/Quartz