Unit 5: Introduction to Metamorphic Petrology

Metamorphism: Agents and Types

Metamorphism is the transformation of a pre-existing rock (the protolith) into a new metamorphic rock. This change occurs in the solid state (without melting) due to changes in temperature, pressure, and/or the introduction of chemically active fluids.

Agents of Metamorphism

1. Temperature (Heat):
   • Provides the energy for chemical reactions that form new minerals.
   • Sources: Geothermal gradient (rocks get hotter with depth) and magma intrusions.
   • This is the most important agent.
2. Pressure:
   • Confining Pressure (or Lithostatic Pressure): Pressure applied equally in all directions, like water pressure. It increases with depth and causes minerals to pack more tightly, forming denser minerals.
   • Directed Stress (or Differential Stress): Pressure that is unequal, with a strong push in one direction. This is common at convergent plate boundaries and causes minerals to align, creating foliation.
3. Chemically Active Fluids (Fluids):
   • Hot water (and dissolved ions) circulating through the rock.
   • These fluids speed up metamorphic reactions by transporting ions, and can change the rock's chemical composition (a process called metasomatism).

Types of Metamorphism

• Contact Metamorphism:
  • Agent: High Temperature (heat from a magma body).
  • Setting: Occurs in the "country rock" surrounding a magma intrusion.
  • Result: Creates a "baked zone" or metamorphic aureole. Rocks are non-foliated (e.g., Hornfels). Pressure is low.
• Regional Metamorphism (Dynamothermal):
  • Agents: High Temperature and High Directed Stress.
  • Setting: Occurs over vast areas (regions) at convergent plate boundaries (mountain-building).
  • Result: Creates foliated rocks (e.g., Slate, Schist, Gneiss). This is the most widespread type of metamorphism.
• Dynamic (Cataclastic) Metamorphism:
  • Agent: High Directed Stress (purely mechanical).
  • Setting: Occurs along fault zones.
  • Result: Rocks are pulverized, crushed, and sheared. Creates rocks like Cataclasite (brittle) or Myllonite (ductile/plastic shear). Temperature is low.
• Other types: Burial Metamorphism (low-grade, from deep burial in a basin), Hydrothermal Metamorphism (heat + hot fluids, common at mid-ocean ridges).

Forms and Structures of Metamorphic Rocks

This refers to the large-scale arrangement of minerals, primarily foliation.

Foliated Structures

Foliation is the parallel alignment of platy minerals (like mica) or the compositional banding of light and dark minerals. It is caused by directed stress and is characteristic of regional metamorphism.

Foliation intensity increases with metamorphic grade (the degree of metamorphism):

1. Slaty Cleavage:
   • Rock: Slate
   • Description: Perfect, flat alignment of microscopic clay and mica crystals. Allows the rock to split into thin, flat sheets.
   • Grade: Low
2. Phyllitic Texture:
   • Rock: Phyllite
   • Description: Fine-grained mica crystals are larger than in slate, giving the rock a wavy or wrinkled appearance and a silky sheen (luster).
   • Grade: Low-to-Medium
3. Schistosity:
   • Rock: Schist
   • Description: Coarse-grained, parallel alignment of large mica crystals (like muscovite, biotite) that are clearly visible. The rock is sparkly.
   • Grade: Medium
4. Gneissic Banding:
   • Rock: Gneiss
   • Description: Coarse-grained rock with compositional banding: alternating light-colored (felsic: quartz, feldspar) and dark-colored (mafic: biotite, amphibole) layers.
   • Grade: High

Non-Foliated Structures

Rocks that lack any alignment of minerals. They form during contact metamorphism (no directed stress) or from protoliths with only one mineral (like quartz or calcite).

• Granoblastic: Composed of interlocking, equigranular crystals (e.g., Quartzite, Marble).
• Hornfelsic: A very fine-grained, dense, and hard texture produced by contact metamorphism (baking).

Textures of Metamorphic Rocks

This refers to the small-scale relationships between mineral grains.

• Porphyroblastic: A large metamorphic crystal (a porphyroblast) grows within a finer-grained matrix (e.g., a large garnet in a schist).
• Poikiloblastic: A porphyroblast that contains numerous inclusions of other small minerals.
• Lepidoblastic: A foliated texture where platy minerals (like mica) are aligned (characteristic of schist).
• Nematoblastic: A foliated texture where needle-like minerals (like amphibole) are aligned (characteristic of amphibolite).
• Granoblastic: (As above) Grains are equidimensional and form a mosaic (characteristic of quartzite, marble).

Nomenclature and Brief Description of Common Metamorphic Rocks

Foliated Rocks (Protolith: Shale)

This is the classic sequence of increasing metamorphic grade from a shale or mudstone protolith:

Shale (Sedimentary) → Slate (Low Grade) → Phyllite → Schist → Gneiss (High Grade) → Migmatite (Partial Melting)

• Slate: Very fine-grained, low-grade, excellent slaty cleavage. Protolith: Shale.
• Phyllite: Fine-grained, silky sheen, wavy foliation. Protolith: Shale/Slate.
• Schist: Medium-to-coarse grained, dominated by visible mica (muscovite, biotite), strong schistosity. Protolith: Shale/Phyllite.
• Gneiss: Coarse-grained, compositional banding (light/dark layers). Protolith: Shale/Schist, or Granite.

Non-Foliated Rocks

• Quartzite: Very hard, dense, crystalline rock. Protolith: Quartz Sandstone. Composed almost entirely of interlocking quartz crystals.
• Marble: Crystalline rock, fizzes with acid. Protolith: Limestone or Dolostone. Composed entirely of interlocking calcite (or dolomite) crystals.
• Hornfels: Very fine-grained, hard, dark-colored rock. "Baked" appearance. Protolith: Any rock (shale, basalt) subjected to contact metamorphism.

Other Important Rocks

• Amphibolite: A medium-to-coarse grained, dark-colored rock composed mainly of Hornblende (Amphibole) and Plagioclase. Often has a nematoblastic texture. Protolith: Basalt or Gabbro.
• Myllonite: Fine-grained, strongly foliated and lineated rock formed by ductile shear in a fault zone (dynamic metamorphism).
• Cataclasite: A broken, fragmented rock (breccia) formed by brittle shear in a fault zone (dynamic metamorphism).
• Migmatite: A composite rock, part metamorphic (Gneiss) and part igneous (Granite). Forms at very high grades when the rock begins to partially melt.
• Granulite: A very high-grade rock (high temp, high pressure) with a granoblastic texture. Protolith: Shale, Basalt.
• Eclogite: A very dense, high-pressure (low temp) rock composed of green Pyroxene (Omphacite) and red Garnet. Protolith: Basalt. Indicates deep subduction.
• Blueschist: A blue-colored rock formed under high pressure but low temperature. The blue color comes from the amphibole Glaucophane. Protolith: Basalt. Also an indicator of subduction zones.