Metamorphism

• Contact Metamorphism:

  • adjacent to igneous intrusions, increasing the temperature in surrounding country rock + local scale (metamorphic aureole)
  • high temperatures (pressure not significant) + minerals not aligned/foliated
  • range of metamorphic rocks formed depending on temperature or distance from intrusion; hornfels-andalusite slate-spotted rock + forms marble/metaquartzite
• Dynamic Metamorphic:

  • fault zones; rocks are crushed/ground into angular fragments by cataclasis in dynamic metamorphic zones at shallow depths
  • directional stress leads to recrystallisation forming smaller, stable, unstrained crystals (eg. fault breccia/mylonite)
• Regional Metamorphism:

  • affects large areas, extending hundred/thousands of km - caused by low-high temperature/pressure at subduction zones or continental collision
  • pressure is significant so minerals have preferred alignment (foliated) - occur in cores of fold mountain belts where mountain ranges have been eroded
  • eg. slate-phylite-schist-gneiss-migmatite
• Index Minerals:

  • chlorite zone represents low grade regional metamorphism (eg. slate) + biotite is low-grade index mineral found in spotted rock)
  • schists develop as increasing temp./pressure; found in biotite/garnet zones (medium grade metamorphism) + schists may have garnet (+kyanite porphyroblasts)
  • quartz/plagioclase feldspar stable throughout all grades (no use as index minerals)

  A) Chlorite

  B) Biotite

  C) Garnet

  D) Kyanite

  E) Silimanite

  
• $Al_2SiO_5$ Polymorphs:

  • kyanite/sillimanite found in regional metamorphosed rocks; kyanite found in gneiss (high-grade) + indicates high pressure, low temperature so not found in contact metamorphism
  • sillimanite zone represents high-grade metamorphism (eg. hornfels; high grade contact metamorphosed rock)
  • andalusite (low-medium temp./low pressure polymorph); found in andalusite slate (medium grade metamorphism)
• Metamorphic Pathways:

  • polymorphism (several crystal structures formed from same chemical elements) - mineral that forms dependant on pressure and temp. conditions
  • different crystal structures more stable at different pressures and temp. - polymorph needing least energy, at given conditions, will be stable
  • more compact/denser crystals structures more stable at higher pressures/larger crystals stable at high temp.
  • polymorphic transformation as slow; polymorph not most stable in current conditions may still exist for some time
• Aluminosilicate ($Al_2SiO_5$) Pathway:

  • aluminosilicate polymorphs common in Al-rich pelitic metamorphic rocks - differences in stability means estimation of pressure/temp. conditions during metamorphism
  • andalusite stable at low pressure, sillimanite at high temp., and kyanite at lower temperatures - move between mineral stable fields as pressure/temp. changes (slow reconstructive transformation into more stable mineral)
  • if pressure/temp. along boundary, both phases are stable + all phases stable at triple point

  
• Aluminosilicate Pathway:

  • andalusite; high temperature, low pressure mineral (medium-high grade contact metamorphism) + kyanite/sillimanite represent regional metamorphism (increase in temp./pressure)
  • changes in mineral composition that occur during burial/heating is prograde metamorphism + changes during uplift and cooling of a rock represent retrograde metamorphism
• Barrow Zones: eg. Dalradian Pelites in Grampians, Scottish Highlands

  • clay-rich sedimentary rocks form variety of metamorphic minerals as pressure/temp. change - Barrow recognised pattern to occurrence of metamorphic minerals; used first appearance of these index minerals to plot isograds/map metamorphic zones
  • Dalradian Pelites divided into 6 zones of metamorphic rocks arranged in increasing grade; first appearance of new mineral marks beginning of new zone

  
• • Isograd - line on a map joining points of equal metamorphic grade/join places where first appearance of index mineral occurs.
  • Metamorphic Zone - area between two isograds named after lower grade isograd; all locations within metamorphic zone experienced same metamorphic grade.