Chptr18

• Metamorphic zones


  Regions bounded by isograd lines that mark the first appearance of a particular index mineral
• Isograd


  A line on a geological map that marks the first appearance of a particular index mineral
• Barrovian metamorphic zones

  • Chlorite zone
  • Biotite zone
  • Almandine (garnet) zone
  • Staurolite zone
  • Kyanite zone
  • Sillimanite zone
• Chlorite zone


  • Bounded by chlorite and biotite isograds
  • Common minerals: chlorite, quartz, muscovite, albite, pyrophyllite
• Biotite zone


  • Bounded by biotite and almandine isograds
  • Common minerals: biotite, quartz, sodium plagioclase, chlorite, muscovite
• Almandine (garnet) zone


  • Bounded by almandine and staurolite isograds
  • Common minerals: almandine garnet, biotite, muscovite, magnetite, quartz, sodium plagioclase
• Staurolite zone


  • Bounded by staurolite and kyanite isograds
  • Common minerals: staurolite, quartz, almandine, potassium feldspar, biotite, muscovite
• Kyanite zone


  • Bounded by kyanite and sillimanite isograds
  • Common minerals: kyanite, biotite, muscovite, almandine garnet, cordierite, quartz
• Sillimanite zone


  • Inside the sillimanite isograd
  • Common minerals: sillimanite, biotite, muscovite, cordierite, quartz, oligoclase, orthoclase
• Barrovian zones and metamorphic isograds remain in use today by geologists studying pelitic metamorphic rocks in the field
• Barrovian zones and isograds are less useful in non-pelitic rocks, or in rocks that form in subduction zone or contact metamorphic environments
• Metamorphic facies


  Distinctive mineral assemblages in metamorphic rocks that form in response to a particular range of temperature and/or pressure conditions
• Metamorphic facies are defined by a group or assemblage of critical minerals, rather than a single index mineral as used for Barrovian zones
• Metamorphic facies do not specifically imply their genetic origin, but the assemblage of minerals within a facies closely constrains the temperature and/or pressure conditions
• Metamorphic facies


  • Sanidinite hornfels
  • Hornblende hornfels
  • Pyroxene hornfels
  • Albite-epidote hornfels
  • Zeolite
  • Prehnite-pumpellyite
  • Greenschist
  • Amphibolite
  • Granulite
  • Blueschist
  • Eclogite
• Hornfels facies


  • Non-foliated, fine-grained or coarser grained with granoblastic textures
  • Form by heat-induced metamorphism in aureoles surrounding igneous intrusions
• Types of hornfels facies

  • Albite-epidote hornfels
  • Hornblende hornfels
  • Pyroxene hornfels
  • Sanidinite hornfels
• Albite-epidote hornfels facies


  Low temperature hornfels facies, with temperatures generally <450°C and pressures <2kbar (depth <6 km)
• Albite-epidote hornfels facies


  • Characteristic minerals are albite and epidote
  • Commonly occur in basaltic tuffs and lavas thermally metamorphosed at ocean ridges, hotspots and in volcanic-magmatic arcs
• Hornblende hornfels facies


  Develop at temperatures generally between 450 and 600°C and at pressures <2.5 kbar (<8 km)
• Pyroxene hornfels facies


  Develop at temperatures of 600-800°C and at pressures <2.5 kbar (<8 km)
• Sanidinite hornfels facies


  Very rare, forming in very high temperature (>800°C) and low pressure (<2.5 kbar = <8 km) conditions
• Zeolite facies


  Low grade metamorphic facies produced by temperatures between ~150 and 300°C and pressures less than 5kbar (~15 km depth)
• Critical zeolite facies minerals


  • Analcime
  • Laumontite
  • Heulandite
  • Wairakite
• Zeolite facies minerals commonly coexist with quartz
• Accessory minerals in the zeolite facies may include albite, kaolinite, vermiculite, adularia, pumpellyite, sphene, epidote, prehnite, montmorillonite, smectite, muscovite, chlorite
• Minerals

  • e
  • hypersthene
  • pigeonite
  • Wollastonite
  • diopside
  • calcium plagioclase (anorthite, bytownite)
• Common rocks

  • Hornfels
  • Hornfels
  • Hornfels
  • Hornfels
  • metaquartzite
  • Marble
  • skarn
• prehnite-pumpellyite minerals form by burial metamorphism in sedimentary basins as well as at ocean ridges, hotspots and volcanic arcs
• Zeolite facies


  Low grade metamorphic facies produced by temperatures between -150 and 300°C and pressures less than 5kbar (-15 km depth)
• Critical zeolite facies minerals


  • analcime
  • laumontite
  • heulandite
  • wairakite
• Accessory minerals in the zeolite facies

  • albite
  • kaolinite
  • vermiculite
  • adularia
  • pumpellyite
  • sphene
  • epidote
  • prehnite
  • montmorillonite
  • smectite
  • muscovite
  • chlorite
  • calcite
• Zeolite facies minerals originate from the hydrothermal alteration of volcanic protoliths such as basalt and andesite, the devitrification of basaltic glass and tuff, and the reaction of pelites and graywackes with saline waters
• Zeolite facies metamorphism

  1. Stilbite heulandite

  2. Laumontite

  3. Wairakite
• Zeolite facies metamorphism develops by hydrothermal alteration at divergent margins, hotspots and convergent margins or during burial metamorphism at depths less than 5 km
• Prehnite-pumpellyite facies


  Low temperature (250-350°C) and fairly low pressure (<6kbar, -20km depth) metamorphic facies
• Common minerals in prehnite-pumpellyite facies

  • quartz
  • albite
  • chlorite
  • muscovite
  • illite
  • phengite
  • smectite
  • sphene
  • titanite
  • epidote
  • lawsonite
  • stilpnomelane
• Protoliths for prehnite-pumpellyite facies

  • basalt
  • graywackes
  • mudstones (pelites)
• Prehnite-pumpellyite facies rocks commonly retain relict textures and structures
• Higher temperature alteration of prehnite and pumpellyite results in the neocrystallization of actinolite and epidote