Unit 5: Metamorphic Petrology

Metamorphism: Definition, Agents, and Factors

Definition of Metamorphism

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 new conditions of temperature, pressure, and/or chemically active fluids.

Agents of Metamorphism

These are the three main drivers of metamorphic change.

1. Temperature (Heat):
   • Provides the energy for chemical reactions that form new, stable minerals.
   • Sources: Geothermal gradient and heat from magma intrusions.
   • This is the most important agent.
2. Pressure:
   • Confining Pressure (Lithostatic): Pressure applied equally in all directions, from the weight of overlying rocks. It causes minerals to pack more tightly, forming denser minerals.
   • Directed Stress (Differential Stress): Pressure that is unequal, with a strong push in one direction (e.g., at convergent plate boundaries). This is the cause of foliation.
3. Chemically Active Fluids (Fluids):
   • Hot water (and dissolved ions) circulating through the rock.
   • These fluids speed up metamorphic reactions by transporting ions.
   • If fluids add or remove elements, the process is called metasomatism.

Factors Controlling Metamorphism

What kind of metamorphic rock you get depends on three main factors:

1. The Protolith (Parent Rock):
   • The original rock's composition determines the composition of the metamorphic rock. You can't make a marble (calcite) from a sandstone (quartz).
   • Protolith → Metamorphic Rock Examples:
     • Shale → Slate → Phyllite → Schist → Gneiss
     • Limestone → Marble
     • Quartz Sandstone → Quartzite
     • Basalt → Amphibolite
2. Temperature and Pressure (Metamorphic Grade):
   • Metamorphic Grade describes the intensity of metamorphism.
   • Low Grade (Low T, Low P): e.g., Slate.
   • Medium Grade: e.g., Schist.
   • High Grade (High T, High P): e.g., Gneiss.
3. Time: Metamorphic reactions are slow, so metamorphism often requires millions of years.

Types of Metamorphism

Metamorphism is classified based on the tectonic setting and the dominant agents.

Structures and Textures of Metamorphic Rocks

These are the key features used to identify metamorphic rocks.

Structures (Large-Scale Features)

The main structural division is foliated vs. non-foliated.

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.

This is a "line-up" of minerals that gets more intense with increasing metamorphic grade:

1. Slaty Cleavage:
   • Rock: Slate
   • Description: Perfect, flat alignment of microscopic clay and mica crystals. Allows the rock to split into thin, flat sheets. (Low Grade).
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.
3. Schistosity:
   • Rock: Schist
   • Description: Coarse-grained, parallel alignment of large mica crystals (like muscovite, biotite) that are clearly visible. The rock is sparkly. (Medium Grade).
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. (High Grade).

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).

• Example Rocks: Marble, Quartzite, Hornfels.

Textures (Small-Scale Features)

These describe the small-scale relationships between mineral grains.

• Granoblastic: Composed of interlocking, equigranular crystals. This is the characteristic texture of Marble and Quartzite.
• Porphyroblastic: A texture where large metamorphic crystals (called porphyroblasts) have grown within a finer-grained matrix (e.g., a large garnet crystal in a schist).
• 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).