Metamorphic Rocks Lecture Notes

Formation of Metamorphic Rocks

• Metamorphic rocks form when parent rocks undergo chemical or physical transformations due to:
  • Increased temperature
  • Increased pressure
  • Interaction with chemically active fluids (e.g., hot water rich in dissolved ions)
• Common geological settings:
  • Areas with raised temperatures and/or pressures
• Metamorphic conditions influence the type of metamorphic rock formed from a parent rock.

Identification and Characteristics

• Textures and Compositions
  • Recognizing metamorphic rock textures and compositions is key to understanding their formation environment.
• Crystal Size
  • Larger crystals in metamorphic rocks can indicate the intensity of the metamorphic conditions.

Non-Foliated Rocks

• Marble
  • Composed primarily of calcite or dolomite.
  • Formed from limestone under increased pressure/temperature without added chemicals.
• Quartzite
  • Formed from quartz-rich parent rocks like quartz sandstone.
  • Crystal size indicates metamorphic grade.
• Scarn
  • Formed in contact metamorphism settings.
  • Contains a mixture of large crystals (calcite, quartz, garnet, epidote).
• Hornfels
  • Formed from mudstone or basalt in contact metamorphism settings.
  • Dark black with a sugary texture.

Foliated Rocks

• Foliation
  • Alignment of crystals under pressure.
  • Extent of foliation and mineral types reveal metamorphic setting.
• Shale, Slate, and Phyllite
  • Shale forms from mudstone under compaction.
  • Slate forms under further pressure and chemical changes.
  • Phyllite has a satiny luster and can contain porphyroblasts.
• Schist
  • Large mica crystals visible, giving a scaly appearance.
• Gneiss
  • Bands of dark and light minerals due to high-grade metamorphism.

Special Cases

• Serpentinite
  • Formed from olivine through hydrothermal metamorphism.
  • Commonly found along subduction zones and transform faults.
• Greenstone and Greenshist
  • Formed from basalt under low-grade metamorphism.
  • Green schist forms with larger crystals under increased pressure.
• Blueschist and Eclogite
  • Formed in subduction zones where pressure exceeds temperature increase.
  • Characterized by specific mineral compositions.

Conclusion

• Identifying metamorphic rocks helps understand their formation history and geological settings.
• Different parent rocks and metamorphic conditions produce a wide variety of metamorphic rocks.
• Knowledge of metamorphic rocks is essential for interpreting geological history and processes.