Themes of the Day:

• Metamorphic Minerals and Textures
• Types of Metamorphic Rocks
• Isograds, Grades, Zones
• Metamorphic Facies

Metamorphic Minerals and Textures

• Minerals:
  • See Table 17-1
  • Examples passed around in class.
• Textures:
  • See Table 18-2; Figures 18-2 to 18-19
  • Examples passed around in class.

Types of Metamorphic Rocks

• Exciting - lots of mineralogical changes due to P, T, fluid conditions
  • Pelites - shale to slate to phyllite to schist to gneiss to migmatite
  • Calc-Silicates (esp. skarns) - "Tremble, for dire peril walks. Monstrous acrimony’s spurning mercy’s laws." --N.L. Bowen, 1938
  • Mafics and Ultramafics - greenschists, blueschists, eclogites (the whole facies diagram)
• Boring - primarily recrystallization
  • sandstone to quartzite
  • pure limestone to marble
  • granitoids to granitic gneisses

Isograds, Grades, Zones

• Metamorphic Grade - general statement of P, T conditions - low, medium, high grade
• Isograds - same grade - generally similar P, T, fluid conditions
• Mineral Isograds - first appearance of a mineral (assumes rocks of same bulk composition)
• Index Minerals (Zones)
  Pelite example (Barrow's Glen Esk Metapelites, Scottish Highlands):
  • LOW GRADE
  • chlorite
  • biotite
  • garnet
  • staurolite
  • kyanite
  • sillimanite
  • sillimanite + K-feldspar
  • HIGH GRADE
• Mapped intersections of reaction front (isograd surface) with topographic surface
• Warning: mineral isograds can cross if bulk composition or fluids vary locally - Whetstone Lake, Canada

Metamorphic Facies

• Originally defined for mafic rocks in Finland by Eskola
• Uniformitarian principle - same bulk composition under same equilibrium conditions will yield the same metamorphic mineral assemblage
• Regions of P,T space
  Examples (note that all are named for metamafic rocks):
  • greenschist
  • amphibolite
  • granulite
  • blueschist
  • eclogite
• Grade (general, relative conditions) vs. Zone (index minerals) vs. Facies (specific P,T regions) - Keep the distinction clear